Make Passphrase "token" credentials accessible via the API

Summary: Fixes T11867. This should really be on the CredentialType itself, but just punt that for now until the API endpoint gets updated. We'll need the actual code here anyway in some form.

Test Plan: {F1922728}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11867

Differential Revision: https://secure.phabricator.com/D16864