Add back 3.0-preview GitLab authz provider

Created by: beyang

This was removed in e9cd0086517d6fd2d069cf6239fdbebd37e21531 in favor of a pure-OAuth-based implementation, but it turns out the old version is needed to satisfy a customer's requirements. Add back the old impl, alongside the OAuth-based impl. The authorization field config object should specify which type ("oauth" or "auth-provider").