Support reading and querying Almanac service PHIDs via "diffusion.repository.search"

Summary:
Ref T13222. See PHI992. If you lose all hosts in a service cluster, you may need to get a list of affected repositories to figure out which backups to pull.

Support doing this via the API.

Test Plan: Queried by service PHID and saw service PHIDs in the call results.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13222

Differential Revision: https://secure.phabricator.com/D19848