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