Improve NetconfDataTreeService to handle RFC6243

RESTCONF is using a generic dispatch to deal with various RFC6243 constructs for both MD-SAL and NETCONF devices.

This is rather unfortunate, because while we have (some) understanding of the MD-SAL datastore model, we have zero understanding as to what the device supports.

Extend NetconfDataTreeService so we can discover whether the device supports :with-defaults capability and expose the appropriate methods to pass through the request, so that RESTCONF can keep its hands clean.