auth: add client_creds_dict

[efiop]

This allows programmatic use without having to create a temporary file.

[efiop]

From what I can see GoogleAuth requires settings file, which is how tests operate https://github.com/iterative/PyDrive2/blob/master/pydrive2/test/test_oauth.py And there is no support for providing settings through a dict, which makes testing this extremely hard without doing awful things like modifying GoogleAuth.DEFAULT_SETTINGS (which we do when using in dvc 🙁 ).