(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.
It might be cleaner to use `update` here, rather than `assoc`.