Allow ConduitClient to use OAuth access tokens for authentication

Summary: Ref T7303. This adds OAuth token support to ConduitClient.

Test Plan: Logged out of an instance and got logged out of the linked upstream; see followup diffs.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T7303

Differential Revision: https://secure.phabricator.com/D15628