Add diffusion.commit.search Conduit API method

Summary: Ref T10978. This is bare bones, but the SearchEngine is at least mostly in reasonable shape now, so get it in place and freeze the old stuff. I previously froze audit.query, which did much the same thing.

Test Plan: Issued some queries with the API, technically got results back.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10978

Differential Revision: https://secure.phabricator.com/D17194