This adds a new authentication mode, Direct Workload Identity
Federation. This new mode permits authenticating to Google Cloud
directly using the GitHub Actions OIDC token instead of proxying through
a Google Cloud Service Account.
There have been a number of GitHub issues recently due to users not adding actions/checkout before calling "auth", which makes the credentials unavailable to future steps. Worse, some people are putting checkout _after_ auth, which overwrites the generated credentials with a checkout of the repo.
This adds a feature that emits a warning with the workspace is empty.