feat: add optional JSON functions crate with json_get_str scaffolding

[crm26]

Summary

Adds a new datafusion-functions-json crate to the workspace with json_get_str as the initial function, establishing the integration pattern for bringing JSON functions into core DataFusion.

Per @alamb's suggestion in #21301:

Perhaps we could make it smaller -- like one that implements scaffolding / new crate / etc and one function. And then we can add the other functions as follow on PRs

What's included

• New crate datafusion/functions-json/ with register_all() hook
• json_get_str(json, *keys) -> str — extracts a string value from a JSON string at the given path (supports nested keys and array indices)
• Optional feature flag json_expressions on the core crate (not enabled by default)
• Handles Utf8, LargeUtf8, and Utf8View string types
• Minimum 2 args enforced at planning time via coerce_types
• sqllogictest coverage (json_functions.slt)
• 9 unit tests + 1 doctest

Design notes

• Uses serde_json (already a workspace dependency) for JSON parsing. The existing datafusion-contrib/datafusion-functions-json crate uses jiter for better performance — follow-on PRs can switch the implementation while keeping the same interface.
• Feature flag is json_expressions, matching the pattern of nested_expressions, unicode_expressions, etc.
• Registration wired through session_state_defaults.rs, same as other function crates.

Follow-on PRs

Will add the remaining functions from datafusion-functions-json: json_get, json_get_int, json_get_float, json_get_bool, json_get_json, json_get_array, json_as_text, json_length, json_contains, and -> / ->> operators.

Ref: #21301