Two small additions to the ES-compat `_mapping(s)` endpoint that
together let downstream callers (e.g. Trino's ES connector) skip the
expensive `list_fields` scan in the common case.

Today `GET /_elastic/{index}/_mapping(s)` calls `list_fields` over every
published split. On indexes with hundreds of thousands of dynamic
fields this can take several seconds and runs into
`QW_FIELD_LIST_SIZE_LIMIT` (100k by default). This PR addresses both
pieces with no proto change:

1. Timestamp pushdown
   New `?start_timestamp=…&end_timestamp=…` URL params on `_mapping(s)`,
   forwarded into `ListFieldsRequest` verbatim. The metastore prunes
   the candidate split set by time window before any leaf fan-out.
   Unit is epoch seconds, half-open interval — matching the existing
   `ListFieldsRequest` proto contract.

2. Column-hints fast path
   New `?fields=…` URL param (comma-separated names). When every
   requested name is a flat literal (no `*`, no `?`, no `.`) declared
   in the union of the indexes' `doc_mapping`, the handler builds the
   response straight from the declared mapping, filtered to those
   names. No `list_fields` call, no split I/O.

   Anything else (wildcards, dotted paths, names not in `doc_mapping`)
   falls through to the full-mapping path: `list_fields` over the
   splits in the time range, full unfiltered mapping returned — same
   shape as today, just with the timestamp-pushdown optimization
   applied.

Notes:
- Unknown query params are silently ignored (no `deny_unknown_fields`)
  to stay compatible with standard ES clients that pass `pretty`,
  `ignore_unavailable`, `allow_no_indices`, etc.
- No proto change. Stays on existing `ListFieldsRequest`.
- `IndexMappingQueryParams` parser and the new
  `ElasticsearchMappingsResponse::from_doc_mapping_filtered` are
  unit-tested in their respective modules.

- drop the fast-path declared-field `retain(...)` in rest_handler.
  `field_patterns` is now hint-only: it triggers the fast path (skip
  `list_fields`) when every pattern matches a flat declared field, and
  is pushed down to the leaves for dynamic-field filtering. Both fast
  and slow paths now return the full declared schema, matching slow-
  path semantics that existed before.
- remove unused `serde_urlencoded` dev-dep from `quickwit-serve` and
  the workspace `Cargo.toml` (was already unused after switching tests
  to `serde_qs`).
- `collect_declared_top_level_names`: switch back to a procedural form
  preallocated with `HashSet::with_capacity(sum)` to signal the upper
  bound — no filtering, no explosion.

- move parse_field_patterns into IndexMappingQueryParams::field_patterns()
- return Vec<String> directly instead of Option<Vec<String>>; caller no
  longer needs unwrap_or_default
- use filter_map to trim and drop empties in one pass
- split final Ok(...) into let response + Ok(response)

Callers using the documented `?fields=` parameter (e.g. the Trino ES
connector) would get an empty ListFieldsRequest.field_patterns, causing
leaves to enumerate all dynamic fields. Adding the serde alias makes
both `?fields=` and `?field_patterns=` map to the same field.

InvalidArgument from root_list_fields (e.g. a pattern with more than
one wildcard) now propagates as an ElasticsearchError instead of being
swallowed by .ok(). Other failures (infrastructure, timeout) still
degrade gracefully to a declared-only mapping response.