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