Add a very basic "auditors" attachment to "differential.commit.search"

Summary: Ref T13631. For now, this only shows the auditor PHID. The current status constants could use some cleanup before they're exposed.

Test Plan: Queried with "auditors" attachment, saw basic auditor information.

Maniphest Tasks: T13631

Differential Revision: https://secure.phabricator.com/D21597