OAuth - add adapter for Phabricator OAuth server
Summary: Ref T887. Pretty straight forward. Got working end to end in Phabricator land.
Test Plan: See next diff of mine, D11287.
Reviewers: epriestley
Reviewed By: epriestley
Subscribers: Korvin, epriestley
Maniphest Tasks: T887
Differential Revision: https://secure.phabricator.com/D11286