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