feat(sink): resolve avro Ref to corresponding Record definition

[xiangjinwu]

I hereby agree to the terms of the RisingWave Labs, Inc. Contributor License Agreement.

What's changed and what's your intention?

This is the sink analogy for #19601 and #19746. Comparing the source and sink support:

• The sink did not support any form of Ref previously, while the source supported a subset (single-layer) via a hack.
• The sink unifies schema and datum handling via MaybeData, while the source took 2 PRs to fix separately.
• The sink uses an owned newtype NamesRef(HashMap<Name, AvroSchema>) while the source uses apache_avro::schema::NamesRef<'s> = HashMap<Name, &'s AvroSchema>. The former requires more memory while the latter requires more time to resolve every time. We can unify to one approach once we learned which is better in practice.
• The sink does not need to reject circular references as source did, because the upstream SQL schema is never circular.

Checklist

• I have written necessary rustdoc comments.
• I have added necessary unit tests and integration tests.
• I have added test labels as necessary.
• I have added fuzzing tests or opened an issue to track them.
• My PR contains breaking changes.
• My PR changes performance-critical code, so I will run (micro) benchmarks and present the results.
• My PR contains critical fixes that are necessary to be merged into the latest release.

Documentation

• My PR needs documentation updates.

Release note

[xiangjinwu]

Ref never resolves to a Union, while a Union may contain a Ref. As a result we check for nullable Union first and then resolve Ref. General (instead of nullability) Union is not supported in sink yet.