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).

[levkk]

You want something like this:

[[write_functions]]
database = "prod"
name = "create_partition"
schema = "partman"

This allows you to disambiguiate between the same function created in different schemas. It's not a common edge case, but it can happen, and we want to be able to scope all Postgres primitives we scan for using a fully qualified identifier. I know that functions can be overloaded, so we should consider capturing parameter data types as well, but for read/write separation it seems unlikely that one version of the function reads and the other one writes, so maybe that's not necessary for this.