Add "PassphraseKey" classes for code which needs to actually use credentials

Summary: Ref T4122. These classes provide typed, checked access to credentials, so you can say "give me this password, and throw if anything is funky".

Test Plan: Used in next revision.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4122

Differential Revision: https://secure.phabricator.com/D7625