Enhancement: Use Google Application Default credentials

Google provide libraries to generate appropriate credentials with various
fallback sources. This change uses the library to generate our GCS client, which
allows us to make use of these extra methods.

This should be backward compatible with previous restic behaviour while adding
the additional capabilities to auth from Google's internal metadata endpoints.
For users running restic in GCP this can make authentication far easier than it
was before.

https://github.com/restic/restic/pull/1552
https://developers.google.com/identity/protocols/application-default-credentials
