chore: Add anonymous credentials json file

[dhpollack]

This allows one to use a credentials json file to create anonymous credentials.

https://googleapis.dev/python/google-auth/latest/_modules/google/auth/credentials.html#AnonymousCredentials

As the docs for AnonymousCredentials themselves states, this only useful for anonymous services and emulators. But it would mean that one could include a credentials file in a project that used an emulator and authenticate in the exact same way as with real credentials. Otherwise one needs to add application logic to allow for anonymous credentials and real credentials.

[conventional-commit-lint-gcf]

🤖 I detect that the PR title and the commit message differ and there's only one commit. To use the PR title for the commit history, you can use Github's automerge feature with squashing, or use automerge label. Good luck human!

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