feat: Include universe_domain in credentials

[dazuma]

Implements the auth requirements for Universe Domain support. Specifically:

• Added a universe_domain attr_accessor to all credentials classes. This was added to the signet monkeypatch for signet-based credentials, to ExternalAccount::BaseCredentials for all the external credential types, and to Googleauth::Credentials as a delegator to the underlying signet client.
• GCECredentials obtains the universe_domain value by looking up the Metadata value
• ExternalAccount types populate the value from the input hash
• JsonKeyReader based credentials (such as ServiceAccount) get the value from the input file.
• Altered the logic for determining whether to use JWT to ensure that JWT is used on non-Google universes.