Add a basic "harbormaster.step.search" API method

Summary: Ref T13585. This isn't particularly useful (notably, it does not include custom field values and isn't searchable by build plan PHID) but get the basics into place.

Test Plan: Used the web UI to make API calls, reviewed results.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13585

Differential Revision: https://secure.phabricator.com/D21488