oauthserver: get client ID/secret from HTTP auth

Summary:
This adds the ability for Phabricator's OAuth server implementation to use HTTP basic auth for the client ID and secret and brings it in line with the OAuth 2.0 specification in this respect.

Fixes T11794

Test Plan: Fixes my use case. Shouldn't impact other use-cases.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: 0, Korvin

Maniphest Tasks: T11794

Differential Revision: https://secure.phabricator.com/D16763