Add a "Token" Credential type

Summary: Ref T9456. This is just a convenience type for things like API tokens, to make it harder for users to make mistakes and keep SSH keys out of the dropdown for "choose your API token".

Test Plan:

Reviewers: chad

Reviewed By: chad

Subscribers: joshuaspence

Maniphest Tasks: T9456

Differential Revision: https://secure.phabricator.com/D14284