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!
Should this be set if we are in delegation mode?