Add a strongly typed internal (Java) RPC API for JSONRestconfService's subscribeToStream()

This is a follow-up to https://git.opendaylight.org/gerrit/#/c/77273/ (re. NETVIRT-1460 & OPNFLWPLUG-1039) :

It seems strange to me that in that PortStatusUpdateInitializer, jhershbe apparently had to use JSONRestconfService's invokeRpc() on sal-remote:create-data-change-event-subscription with a SUBSCRIBE_JSON String.

We'll merge that, but wouldn't be better if there was a clean strongly typed internal (Java) API for that instead of this weird architecture where neutron uses restconf to invoke an RPC like this?

JMorvay is this something you would consider adding?