Put some glue in between PhabricatorAuthProvider and the OAuth adapters

Summary: Ref T1536. None of this code is reachable. Glues AuthProvider to OAuthAdapter.

Test Plan: Code is unreachable.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1536

Differential Revision: https://secure.phabricator.com/D6160