Implement passphrase.query for querying credentials
Summary: Resolves T5868. This implements passphrase.query and a mechanism for allowing Conduit access to credentials.
Test Plan: Tested locally.
Reviewers: epriestley, #blessed_reviewers
Reviewed By: epriestley, #blessed_reviewers
Subscribers: talshiri, epriestley, Korvin
Maniphest Tasks: T5868
Differential Revision: https://secure.phabricator.com/D10262