Add auth.querypublickeys to retrieve public keys

Summary:
Fixes T6484. I primarily need this to synchronize device public keys in the Phabricator cluster so the new stuff in T2783 works.

Although, actually, maybe I don't really need it. But I wrote it anyway and it's desirable to have sooner or later.

Test Plan: Ran method.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6484

Differential Revision: https://secure.phabricator.com/D11163