…opt (#2544)

This fixes a bug in how the auth proxy handled the configuration when the `--impersonate-service-account` flag and the `auto-iam-auth` instance configuration parameter are set at the same time. This is especially important to make the Cloud SQL Proxy Operator work correctly, see GoogleCloudPlatform/cloud-sql-proxy-operator#719

Fixes #2542

This change ensures that the universe domain configuration is correctly passed when setting up an impersonation chain for credentials.

It also introduces internal test infrastructure to mock `impersonate.CredentialsTokenSource` and adds a unit test `TestCredentialsOpt` to verify this behavior.

Usage: ./build.sh build_image {image_url}

This builds the cloud-sql-proxy binary from the local source code and pushes it to 
a container registry.

Co-authored-by: Ben Knutson <benknutson@google.com>

* chore(main): release 2.21.1

* Updates license check file with new version number.

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Co-authored-by: release-please[bot] <55107282+release-please[bot]@users.noreply.github.com>
Co-authored-by: Jonathan Hess <hessjc@google.com>