Load table credentials from credentials endpoint

[kevinjqliu]

Summary

• Load table credentials from /credentials during REST table response handling when inline credentials are absent.
• Only call the endpoint when the catalog advertises loadCredentials and vended-credentials access delegation is requested.
• Resolve loaded storage credentials by longest prefix against the table location, while keeping inline storage-credentials precedence over catalog config.
• Avoid the extra /credentials request for remote signing and catalogs that do not advertise the endpoint.

Stacked on #3499.

Testing

• PYTHONPATH=. pytest tests/catalog/test_rest.py -k 'load_credentials or storage_credentials or load_table_honor_access_delegation or load_table_without_storage_credentials'
• ruff check pyiceberg/catalog/rest/__init__.py tests/catalog/test_rest.py
• ruff format --check pyiceberg/catalog/rest/__init__.py tests/catalog/test_rest.py
• pre-commit hooks during git commit