feat(google-auth): grpc cert rotation handling

[agrawalradhika-cell]

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Fixes #<issue_number_goes_here> 🦕

[gemini-code-assist]

Code Review

This pull request introduces a mechanism for mTLS certificate rotation in gRPC channels by implementing a refreshing channel wrapper and a call interceptor. The review identified several issues: a missing logger definition causing potential runtime errors, a typo in a variable name, redundant parameters in the interceptor, the use of print statements instead of logging, and a lack of support for non-unary gRPC call types and context manager patterns.

[gemini-code-assist]

The _MTLSCallInterceptor currently only implements intercept_unary_unary. To fully support certificate rotation for all gRPC call types, it should also implement intercept_unary_stream, intercept_stream_unary, and intercept_stream_stream.

[gemini-code-assist]

_MTLSRefreshingChannel should implement __enter__ and __exit__ to support being used as a context manager, which is a standard pattern for gRPC channels.

    def __init__(self, target, factory_args, initial_channel, initial_cert):
        self._target = target
        self._factory_args = factory_args
        self._channel = initial_channel
        self._cached_cert = initial_cert
        self._lock = threading.Lock()

    def __enter__(self):
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        self.close()