Support unauthenticated clients via Unauthenticated auth source

This change set introduces a new Unauthenticated global variable that is an instance of the new unauthenticatedAuthSource that can be used to signal that the client should not use any authentication. I've preferred that over using nil for the auth source, because I simply don't like passing nil for something I intentionally want and a personal dislike towards nil :)

I think it also makes the API more expressive.