feat: add configurable write_functions

[murex971]

Description

• Implemented configurable write_functions routing in the new parser path.
• Added optional schema support so functions can be fully qualified per DB.
• For configured entries, matching ollows PostgreSQL identifier behavior (unquoted -> lowercase, quoted -> case-sensitive).
• Kept existing hardcoded write functions.

Testing

Added unit and integration coverage for routing, schema qualification, and PG identifier semantics.

Related Issues

#623

[murex971]

@levkk should new features like these be compatible with legacy parser path too ?
currently added to new parser path only.

[codecov]

Codecov Report

❌ Patch coverage is 96.75325% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pgdog/src/backend/pool/cluster.rs	89.79%	5 Missing ⚠️

📢 Thoughts on this report? Let us know!

[levkk]

Functions need to be fully qualifiable, e.g.: partman.create_partition is a write function, while create_partition(array) may not be.

[murex971]

added optional schema, not sure if you were referring to this ?

[levkk]

@levkk should new features like these be compatible with legacy parser path too ? currently added to new parser path only.

That's fine, we'll be removing the legacy parser shortly (tm).

[murex971]

@levkk I noticed a behavior gap around identifier normalization from pgdog.toml vs PostgreSQL semantics.

Right now, for write_functions, we normalize config identifiers with PG-like rules (unquoted -> lowercase, quoted -> case-preserving), but this is not consistently applied across other config-driven identifier matching paths in PgDog.