feat(iam): support an alternative env to decide if mtls should be enabled

[amtk3]

CLOUDSDK_CONTEXT_AWARE_USE_CLIENT_CERTIFICATE is another endpoint that can be set in Gcloud CLI to enable Certificate Based Access. We should support it as well.

[gemini-code-assist]

Summary of Changes

Hello @amtk3, 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 IAM authentication logic by incorporating an additional environment variable to control the enablement of client certificates for mutual TLS. This change ensures broader compatibility with Gcloud CLI configurations for certificate-based access, offering more flexibility in how mTLS is activated within the system.

Highlights

• Environment Variable Support: The pull request introduces support for the CLOUDSDK_CONTEXT_AWARE_USE_CLIENT_CERTIFICATE environment variable.
• mTLS Enablement: This new environment variable provides an alternative mechanism to enable mTLS (mutual TLS) for certificate-based access, aligning with Gcloud CLI's configuration options.

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[gemini-code-assist]

Code Review

This pull request adds support for the CLOUDSDK_CONTEXT_AWARE_USE_CLIENT_CERTIFICATE environment variable to enable mTLS, which is a good addition. My review includes a suggestion to refactor the logic for checking environment variables to improve readability and maintainability.

A few additional points for consideration:

• It would be best to define CLOUDSDK_CONTEXT_AWARE_USE_CLIENT_CERTIFICATE as a constant in google/auth/environment_vars.py and import it to avoid using a magic string and maintain consistency with other environment variables.
• While the changed code is marked with pragma: NO COVER, consider adding a test case for this fallback logic to prevent future regressions. This would likely involve mocking hasattr(mtls, 'should_use_client_cert') to return False and then testing the behavior with different environment variable settings.