REP-1271 Implement Delegation in Client AuthN

So the only thing I don't like about this change is that the message is not very descriptive. Unfortunately, to provide a more descriptive message I would have to spend a fair amount of time refactoring the authentication filter. Options there include returning an object that includes a message, using ThreadLocal, or refactoring the code to be less distributed so that setting the header alongside the status code makes sense.

I also did not run the system test suite locally, so some of those may not pass. Let's find out!
It doesn't really matter if it is or isn't set. It won't be returned, and can be overwritten later. I believe this is safe, but I could add logic to only change the response status if not delegating.