(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.
I think I might just use keywords for these, rather than strings.  e.g. `:context`, `:ring`, etc.  And in that case it might not be worth defining constants for them.