[SPARK-49967][SQL] Codegen Support for `StructsToJson`(`to_json`)

[panbingkun]

What changes were proposed in this pull request?

The pr aims to add Codegen Support for StructsToJson(to_json).

Why are the changes needed?

• improve codegen coverage.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

Pass GA & Existed UT (eg: JsonFunctionsSuite#*to_json*)

Was this patch authored or co-authored using generative AI tooling?

No.