Multi-rack: need DDM API endpoint for originating rack prefixes #939

We need to add an API endpoint for omicron to poke ddmd in the switch zone (TransitRouter) to start/stop/update originating a route for the /56 rack prefix.

This route should have the ability to carry an optional SockAddr (SockAddrJson?) that holds the IP/port of the sled-agent within this rack which is running the multi-rack join service.