R1:5d30ea56cf5e

Add a modern `user.search` Conduit API method

Summary: Ref T10512. This is fairly bare-bones but appears to work.

Test Plan: Queried all users, queried some stuff by constraints.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10512

Differential Revision: https://secure.phabricator.com/D15959