(TK-10) Add get-registered-endpoints function with tests

I've added the get-registered-endpoints function to the service and have written some tests to go along with it. Whenever one of the service functions is called, a set in the state is updated with a map containing information on the endpoint and what created that endpoint. This set is then returned by the get-registered-endpoints function.
You've been consistent with the term `:endpoint` in all of the previous functions, so I think we should keep doing that here.  So maybe change `old-prefix` to `endpoint`, and `new-prefix` to `target-path`?  Also maybe change `host` and `port` to `target-host` and `target-port`, just to be explicit?  That might be overkill though, maybe someone else can weigh in on that.