Disambiguate s3 and gs anonymous auth

[thobrla]

Initially reported in this StackOverflow post.

Presently, ConfigureNoOpAuthIfNeeded is only called when no credentials are available for s3 or GCS. However, it is possible that a user may want to access one anonymously while using their credentials for the other.

We should have separate no-op auth handlers for both s3 and gs capabilities so that they can be used when necessary.

This will likely require refactoring of the auth helper logic in gslib/util.py, as we want to avoid boto.auth.get_auth_handler connecting to a service endpoint unnecessarily.

[jhoareau]

This issue is still relevant for me. Has anyone ever looked into it?