Allow custom endpoint override with ENDPOINT_TYPE

[karubian]

When both ENDPOINT_TYPE and ENDPOINT are provided, ENDPOINT_TYPE silently overwrites the user-provided ENDPOINT. This means there's no way to get the catalog-specific defaults from ENDPOINT_TYPE (auth type, path prefix, supported operations) while pointing at a different host.

Current behavior

sql
ATTACH 'my-warehouse' AS catalog (
   TYPE iceberg,
   ENDPOINT_TYPE glue,
   ENDPOINT 'my-custom-host.example.com/iceberg'
);

-- User-provided ENDPOINT is silently ignored
-- Connects to the hardcoded default host instead

Proposed behavior

If the user explicitly provides ENDPOINT, respect it. ENDPOINT_TYPE should still apply its defaults (auth, path prefix, catalog-specific settings) but not overwrite the host.

sql
ATTACH 'my-warehouse' AS catalog (
   TYPE iceberg,
   ENDPOINT_TYPE glue,
   ENDPOINT 'my-custom-host.example.com/iceberg'
);

-- Connects to my-custom-host.example.com/iceberg
-- Auth, path prefix, and catalog defaults from ENDPOINT_TYPE still apply

Use cases

• VPC endpoints / PrivateLink
• FIPS endpoints
• Region-specific hostname variations (related: missing '.cn' in hostname when attaching to aws s3 table bucket #708)
• Local development with endpoint emulators