refactor: decouple AuthManager from pg_catalog

[iPeluwa]

Fixes #191

This change decouples AuthManager from PgCatalogContextProvider to unblock issue #189 (moving pg_catalog to a separate module).

Changes:

• Add RoleProvider trait for providing role information to pg_catalog
• Implement RoleProvider for AuthManager and Arc
• Add RoleProviderBridge adapter that wraps RoleProvider and implements PgCatalogContextProvider - this bridges the two worlds without tight coupling
• Remove direct PgCatalogContextProvider impl from AuthManager
• Update testing.rs and CLI to use RoleProviderBridge

The architecture now looks like:
AuthManager -> RoleProvider trait RoleProviderBridge -> PgCatalogContextProvider trait pg_catalog uses PgCatalogContextProvider for pg_roles table

This keeps auth concerns separate from pg_catalog while still allowing role information to flow to the pg_roles table.

[iPeluwa]

@sunng87 Could you please review this ?

[sunng87]

@iPeluwa I thought I comment about this but I forgot to hit send button. I think this has some overlap with PgCatalogContextProvider. The idea of PgCatalogContextProvider is to decouple AuthManager with our pg_catalog. It's already done. I don't we need one more layer between them.

[iPeluwa]

@iPeluwa I thought I comment about this but I forgot to hit send button. I think this has some overlap with PgCatalogContextProvider. The idea of PgCatalogContextProvider is to decouple AuthManager with our pg_catalog. It's already done. I don't we need one more layer between them.

Thanks for the feedback! You're right - I see now that PgCatalogContextProvider was already designed as the abstraction layer to decouple auth from pg_catalog.

Looking at it again, the actual coupling I was trying to address was that AuthManager in datafusion-postgres had to directly import from datafusion-pg-catalog crate to implement
PgCatalogContextProvider. But that's a compile-time dependency, not a tight coupling issue.

The PgCatalogContextProvider trait itself is already the abstraction - any type can implement it and be passed to setup_pg_catalog(). My extra RoleProvider layer just adds unnecessary indirection.

@sunng87 please tag me to issues that needs to a addressed.