feat: Auto enable mTLS when supported certificates are detected

[agrawalradhika-cell]

The Python SDK will use a hybrid approach for mTLS enablement:

If the GOOGLE_API_USE_CLIENT_CERTIFICATE environment variable is set (either true or false or any value), the SDK will respect that setting. This is necessary for test scenarios and users who need to explicitly control mTLS behavior.
If the GOOGLE_API_USE_CLIENT_CERTIFICATE environment variable is not set, the SDK will automatically enable mTLS only if it detects Managed Workload Identity (MWID) or X.509 Workforce Identity Federation (WIF) certificate sources. In other cases where the variable is not set, mTLS will remain disabled.

[gemini-code-assist]

Summary of Changes

Hello @agrawalradhika-cell, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the Google Cloud Python SDK's handling of mutual TLS (mTLS) by introducing a more sophisticated and adaptive enablement mechanism. It prioritizes explicit user control through an environment variable while also enabling mTLS automatically in secure environments where specific certificate sources are detected. This change aims to improve security posture by default for supported authentication methods without requiring manual configuration, while maintaining flexibility for users who need precise control.

Highlights

• Hybrid mTLS Enablement: Implements a new hybrid strategy for enabling mutual TLS (mTLS) in the Python SDK, balancing explicit user control with automatic detection.
• Environment Variable Priority: The GOOGLE_API_USE_CLIENT_CERTIFICATE environment variable now explicitly controls mTLS behavior if it is set (to any value).
• Automatic Detection: If the environment variable is not set, mTLS is automatically enabled only when Managed Workload Identity (MWID) or X.509 Workforce Identity Federation (WIF) certificate sources are detected.
• Default Disabled: mTLS remains disabled by default if the environment variable is not set and no supported certificate sources are found.
• Refactored Logic: Introduced a new static method _use_client_cert_effective to centralize the mTLS enablement decision logic, leveraging google-auth's should_use_client_cert for automatic detection.

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[gemini-code-assist]

Code Review

This pull request introduces logic to automatically enable mTLS when supported certificates are detected, falling back to environment variables for older google-auth versions. The changes are mostly good refactorings to centralize this logic. However, I found a critical bug in google/cloud/storage/client.py where a module is used without being imported, which will lead to a NameError. I've suggested a fix for the import and also a refactoring to simplify some duplicated code in the same file.

[chandra-siri]

kokoro failure should be fixed once this is merged - #1635

[daniel-sanche]

You can ignore my comments on the auto-generated code, those should have been left in the gapic-generator PR

Hand-wrritten changes LGTM