Add "bin/auth revoke --list" to explain what can be revoked

Summary:
Depends on D18908. Ref T13043. Allow users to get information about what revokers do with a new --list flag.

You can use --list --type <key> to get information about a specfic revoker.

Test Plan: Ran bin/auth revoke --list, saw a list of revokers with useful information.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13043

Differential Revision: https://secure.phabricator.com/D18910