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