Get project id from google.auth.default() in empty GcpCredentials block if quota project is None

[kevingrismore]

Adds a check to GcpCredentials block initialization to acquire the GCP project id from google.auth.default() if no quota_project_id exists. This scenario arises when initializing an empty block on compute in Cloud Run, Compute Engine, or App Engine in which gcloud auth application-default login has not been run. Enables Cloud Run workers on compute with an attached service account and the correct roles to start Cloud Run jobs without requiring a completed GCP Credentials block to be added to the connected work pool.

Example

Screenshots

Checklist

• References any related issue by including "Closes #" or "Closes ".
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• Includes tests or only affects documentation.
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[desertaxle]

Thanks for the PR @kevingrismore! Could you please write a test covering the new condition? Let me know if you need any help!

[kevingrismore]

@desertaxle Added a test, but I haven't worked with MagicMock before so I tried to model it after an existing test. Open to any recommendations for improvement!

[desertaxle]

LGTM!