feat: Add mTLS Interceptor and Channel Wrapper for certificate rotation handling

[agrawalradhika-cell]

Implement mTLS Interceptor and Channel Wrapper for automatic certificate rotation and retry logic.

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Fixes #<issue_number_goes_here> 🦕

[gemini-code-assist]

Code Review

This pull request introduces a new mTLS interceptor and channel wrapper (mtls_interceptor.py) to handle automatic retry logic and certificate rotation for gRPC connections. The review feedback highlights several critical runtime and import-time issues that must be addressed, including missing imports for _LOGGER and _mtls_helper, an undefined parameter create_channel_fn in MTLSRefreshingChannel.__init__, an immediate overwrite of self._channel using an undefined secure_authorized_channel function, and a NameError caused by referencing _BaseCallWrapper as a base class before its definition.

[gemini-code-assist]

The newly created channel assigned to self._channel on line 124 is immediately overwritten on line 130 by a call to secure_authorized_channel(**factory_args). Furthermore, secure_authorized_channel is not defined or imported in this module. Please correct this logic to ensure the channel is properly initialized and not overwritten.

References

1. Before flagging a potential NameError or suggesting a fully-qualified import path in a code review, verify all module-level imports to ensure the module or variable is not already imported.

[gemini-code-assist]

The class _BaseCallWrapper is used as a base class for _RetryableUnaryResponseFuture before it is defined (it is defined on line 662). In Python, base classes must be defined before they are referenced in a class definition, otherwise a NameError is raised at import time. Please move the definition of _BaseCallWrapper above _RetryableUnaryResponseFuture.

References

1. Before flagging a potential NameError or suggesting a fully-qualified import path in a code review, verify all module-level imports to ensure the module or variable is not already imported.

[gemini-code-assist]

The module uses _LOGGER and _mtls_helper but does not import or define them, which will cause NameError exceptions at runtime. Please import logging and _mtls_helper, and initialize _LOGGER.

import collections
import logging
import threading
import time

import grpc
from google.auth import transport
from google.auth.transport import _mtls_helper

_LOGGER = logging.getLogger(__name__)

References

1. Before flagging a potential NameError or suggesting a fully-qualified import path in a code review, verify all module-level imports to ensure the module or variable is not already imported.