Include service name in get_credential return

The object returned by get_credential currently includes the username and password, but doesn't include the service name itself. A get_credential().service attribute would be helpful when multiple credential objects are passed to another function or variable.

Start at the get_credential entry point and inspect the returned credential object's existing username and password fields. Done means the returned object also exposes the service name while preserving its current fields; verify this against the project's existing credential behavior.

Written by the indexing model from the issue text.