feat: UNNEST with ordinality and offset support

[MartinSahlen]

Which issue does this PR close?

• Closes #.

Rationale for this change

UNNEST as a table factor previously rejected both WITH ORDINALITY (Postgres / SQL standard) and WITH OFFSET (BigQuery) with not_impl_err!. Both spellings express the same semantic need — emit a per-element position
alongside the unnested value — and the SQL parser (sqlparser-rs) already produces a single AST that distinguishes them by keyword.

Supporting both is justified by:

• They're syntactic siblings, not alternatives. sqlparser-rs parses both into the same TableFactor::UNNEST. The physical execution and logical-plan shape are identical modulo a constant index base.
• No SQL dialect supports both at once; each dialect picks one. Accepting both makes DataFusion a clean target for queries written against either Postgres/Trino (WITH ORDINALITY, 1-indexed) or BigQuery (WITH OFFSET,
  0-indexed), with no rewriting required.
• The two keywords carry their standard semantics, not a configurable flag: WITH ORDINALITY is always 1-indexed (SQL:2003), WITH OFFSET is always 0-indexed (BigQuery). This mirrors BigQuery's own precedent for array
  indexing (arr[OFFSET(0)] vs arr[ORDINAL(1)]) — keyword carries the semantics, no surprise.
• The two are mutually exclusive in the same statement; the planner rejects them combined.

What changes are included in this PR?

• datafusion-common: IndexBase enum (Zero / One), PositionColumn { name, base }, UnnestOptions.position: Option, builder method with_position.
• Logical plan: Unnest::try_new appends a nullable Int64 field to the output schema when options.position is set.
• SQL planner (relation/mod.rs): handles WITH ORDINALITY and WITH OFFSET [alias], defaults position column to "ordinality" / "offset", rejects the both-at-once combination. Postgres-style AS t(v, ord) column-list aliasing
  works through the existing alias mechanism.
• try_process_unnest threads the position option through to unnest_columns_with_options and projects the position column on the outer SELECT.
• Physical plan (UnnestExec): create_position_indices materializes the position column at the leaf unnest level, using the supplied IndexBase.
• Proto: new IndexBase enum and PositionColumn message; UnnestOptions.position round-trips end-to-end through to_proto / from_proto.
• Not included: unparser support (kept best-effort; can be added later based on feedback).

Are these changes tested?

Yes:

• datafusion/sqllogictest/test_files/unnest.slt: execution result tests for WITH OFFSET, WITH OFFSET , WITH ORDINALITY, WITH ORDINALITY AS t(v, ord), plus both error cases.
• datafusion/physical-plan/src/unnest.rs: unit tests for create_position_indices (0-indexed and 1-indexed).
• datafusion/proto/tests/cases/roundtrip_logical_plan.rs: logical-plan proto round-trip tests for both spellings.
• The prior expected-failure cases at unnest.slt:425-430 are now positive cases.

Are there any user-facing changes?

Yes — the SQL surface accepts UNNEST(...) WITH ORDINALITY [AS t(value_alias, ord_alias)] and UNNEST(...) WITH OFFSET [alias] in the FROM clause. The UnnestOptions public type gains a position field (additive, default
None); existing DataFrame callers are source-compatible.

[github-actions]

Thank you for opening this pull request!

Reviewer note: cargo-semver-checks reported the current version number is not SemVer-compatible with the changes in this pull request (compared against the base branch).

Details

     Cloning apache/main
    Building datafusion-common v53.1.0 (current)
       Built [  35.770s] (current)
     Parsing datafusion-common v53.1.0 (current)
      Parsed [   0.060s] (current)
    Building datafusion-common v53.1.0 (baseline)
       Built [  31.541s] (baseline)
     Parsing datafusion-common v53.1.0 (baseline)
      Parsed [   0.060s] (baseline)
    Checking datafusion-common v53.1.0 -> v53.1.0 (no change; assume patch)
     Checked [   0.895s] 222 checks: 221 pass, 1 fail, 0 warn, 30 skip

--- failure constructible_struct_adds_field: externally-constructible struct adds field ---

Description:
A pub struct constructible with a struct literal has a new pub field. Existing struct literals must be updated to include the new field.
        ref: https://doc.rust-lang.org/reference/expressions/struct-expr.html
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.47.0/src/lints/constructible_struct_adds_field.ron

Failed in:
  field UnnestOptions.position in /home/runner/work/datafusion/datafusion/datafusion/common/src/unnest.rs:84

     Summary semver requires new major version: 1 major and 0 minor checks failed
    Finished [  70.232s] datafusion-common
    Building datafusion-expr v53.1.0 (current)
       Built [  25.156s] (current)
     Parsing datafusion-expr v53.1.0 (current)
      Parsed [   0.075s] (current)
    Building datafusion-expr v53.1.0 (baseline)
       Built [  25.146s] (baseline)
     Parsing datafusion-expr v53.1.0 (baseline)
      Parsed [   0.077s] (baseline)
    Checking datafusion-expr v53.1.0 -> v53.1.0 (no change; assume patch)
     Checked [   1.706s] 222 checks: 222 pass, 30 skip
     Summary no semver update required
    Finished [  54.090s] datafusion-expr
    Building datafusion-physical-plan v53.1.0 (current)
       Built [  31.056s] (current)
     Parsing datafusion-physical-plan v53.1.0 (current)
      Parsed [   0.129s] (current)
    Building datafusion-physical-plan v53.1.0 (baseline)
       Built [  31.083s] (baseline)
     Parsing datafusion-physical-plan v53.1.0 (baseline)
      Parsed [   0.132s] (baseline)
    Checking datafusion-physical-plan v53.1.0 -> v53.1.0 (no change; assume patch)
     Checked [   0.844s] 222 checks: 222 pass, 30 skip
     Summary no semver update required
    Finished [  65.478s] datafusion-physical-plan
    Building datafusion-proto v53.1.0 (current)
       Built [  52.352s] (current)
     Parsing datafusion-proto v53.1.0 (current)
      Parsed [   0.142s] (current)
    Building datafusion-proto v53.1.0 (baseline)
       Built [  52.127s] (baseline)
     Parsing datafusion-proto v53.1.0 (baseline)
      Parsed [   0.142s] (baseline)
    Checking datafusion-proto v53.1.0 -> v53.1.0 (no change; assume patch)
     Checked [   2.463s] 222 checks: 221 pass, 1 fail, 0 warn, 30 skip

--- failure constructible_struct_adds_field: externally-constructible struct adds field ---

Description:
A pub struct constructible with a struct literal has a new pub field. Existing struct literals must be updated to include the new field.
        ref: https://doc.rust-lang.org/reference/expressions/struct-expr.html
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.47.0/src/lints/constructible_struct_adds_field.ron

Failed in:
  field UnnestOptions.position in /home/runner/work/datafusion/datafusion/datafusion/proto/src/generated/prost.rs:538
  field UnnestOptions.position in /home/runner/work/datafusion/datafusion/datafusion/proto/src/generated/prost.rs:538

     Summary semver requires new major version: 1 major and 0 minor checks failed
    Finished [ 110.120s] datafusion-proto
    Building datafusion-sql v53.1.0 (current)
       Built [  38.989s] (current)
     Parsing datafusion-sql v53.1.0 (current)
      Parsed [   0.031s] (current)
    Building datafusion-sql v53.1.0 (baseline)
       Built [  38.759s] (baseline)
     Parsing datafusion-sql v53.1.0 (baseline)
      Parsed [   0.033s] (baseline)
    Checking datafusion-sql v53.1.0 -> v53.1.0 (no change; assume patch)
     Checked [   0.315s] 222 checks: 222 pass, 30 skip
     Summary no semver update required
    Finished [  79.826s] datafusion-sql
    Building datafusion-sqllogictest v53.1.0 (current)
       Built [ 165.278s] (current)
     Parsing datafusion-sqllogictest v53.1.0 (current)
      Parsed [   0.022s] (current)
    Building datafusion-sqllogictest v53.1.0 (baseline)
       Built [ 165.332s] (baseline)
     Parsing datafusion-sqllogictest v53.1.0 (baseline)
      Parsed [   0.023s] (baseline)
    Checking datafusion-sqllogictest v53.1.0 -> v53.1.0 (no change; assume patch)
     Checked [   0.114s] 222 checks: 222 pass, 30 skip
     Summary no semver update required
    Finished [ 334.648s] datafusion-sqllogictest