Maniphest - add conduit method to get status information

Summary: Ref T4938. `arc close` needs to know about custom statuses and this conduit method is step 1 of letting it know

Test Plan: See next diff, which works!

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T4938

Differential Revision: https://secure.phabricator.com/D8937