(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.
This code is pretty WET.  What do you think about introducing a `core/register-endpoint` function that would take `state` and the endpoint type (`:context` here) and handle constructing the map and updating the state, based on which type of endpoint it was handling?  (and then use that to DRY up this function and the ones below)