Add "auditors" filter to User API

Proposal

It is possible for admin users to filter users returned by the users API to just those users with admin rights (?admins=true).

There doesn't appear to be a way to do this for auditors.

Adding in an ?auditors=true option would be helpful for us.

Use case: A site gives auditor access to certain personnel and manages this access programmatically via the API. Current workaround is just to iterate through the entire GitLab userbase and inspect the is_auditor attribute for each user in order to remove auditor access for personnel that leave.