feat(auth): add mTLS helper methods and endpoint configuration for ha…

[nbayati]

This PR builds on top of PR googleapis#16976. It provides helper methods to allow custom HTTP and WebSocket connection pools (such as those in google-genai and google-adk) to load default client certificates and resolve the GOOGLE_API_USE_MTLS_ENDPOINT env var. Changes include:

• Introduced GOOGLE_API_USE_MTLS_ENDPOINT environment variable to control whether an mTLS endpoint should be used (always, never, or auto).
• Added several new helper functions in google.auth.transport.mtls to facilitate SSL context creation and client certificate loading:
  • load_client_cert_into_context: Loads a client certificate and key into a provided SSL context.
  • make_client_cert_ssl_context: Creates a default SSL context loaded with a specific client certificate and key.
  • load_default_client_cert: Discovers and loads the default client certificate into a provided SSL context if mTLS is enabled.
  • get_default_ssl_context: Returns a default SSL context pre-loaded with the default client certificate, or None if unavailable.
  • should_use_mtls_endpoint: Determines if an mTLS endpoint should be used based on the new environment variable and certificate availability.
• Fixed outdated docstrings for default_client_cert_source and default_client_encrypted_cert_source to correctly state they raise MutualTLSChannelError instead of DefaultClientCertSourceError.
• Updated default_client_cert_source to also catch ClientCertError when loading credentials.
• Added comprehensive unit tests for the new mTLS helper methods.

[nbayati]

closing as the base has to be upstream not my fork and it cannot be changed.