Add a rough harbormaster.querybuildables Conduit API method

Summary: Ref T4809. I need to sort out some of the "status" stuff we're doing before this is actually useful (there's no sensible "status" value to expose right now) but once that happens arc can query this to figure out whether it needs to warn the user about pending/failed builds.

Test Plan: Ran query with various different parameters.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4809

Differential Revision: https://secure.phabricator.com/D8794