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