feat: Add PSC DNS and Global Write Endpoint support to Python Connector

[hessjcg]

Add PSC DNS and Global Write Endpoint support to Python Connector

• Implement resolve_cname and resolve_txt async wrappers inside DnsResolver.
• Implement SetClient in DnsResolver to inject the lazy-loaded CloudSQLClient.
• Update DnsResolver.resolve() to intercept direct PSC DNS names and call resolve_connect_settings() API.
• Handle CNAME recursive resolution loop recursion-free (max 10 depth) inside resolve(), checking loop circularity.
• Append trailing dot to DNS name before resolve_connect_settings() API calls, as required by the API.
• Implement resolve_connect_settings() inside CloudSQLClient to fetch metadata on ConnectSettings resolve REST endpoint.
• Implement _get_fallback_ip() and _is_ip_address() helpers in connector.py.
• Update connector.py connect_async() to support fallback to Private/Primary (Public) IP from metadata if CNAME/PSC DNS resolution fails and metadata preferred address is a hostname.
• Add comprehensive unit tests for direct PSC DNS, simple CNAME to PSC, recursive CNAMEs, and CNAME loop detection in test_resolver.py.
• Add test_Connector_connect_async_custom_dns_resolver_fallback_psc_to_private_ip unit test to test_connector.py.

TAG=agy
CONV=0b6ef4f2-88db-48b8-9bb9-54cd56459291