feat: add configurable write_functions

[murex971]

Description

• Implemented configurable write_functions routing in the new parser path.
• 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 96.51163% with 6 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pgdog/src/backend/pool/cluster.rs	91.66%	6 Missing ⚠️

📢 Thoughts on this report? Let us know!

[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).