R1:cefbdbcffe9e

Provide a "Reviewers" attachment to "differential.revision.search"

Summary: Allow API callers to retrieve reviewer information via a new "reviewers" attachment.

Test Plan: {F4675784}

Reviewers: chad, lvital

Reviewed By: lvital

Subscribers: lvital

Differential Revision: https://secure.phabricator.com/D17633