Create an internal service to access restconf

We have a use case where we want to access restconf internally to take advantage of the URL path and JSON parsing and the mount point invocation functionality. Rather than duplicating all that code internally, we want to reuse the restconf code.

We could access restconf via HTTP, as others have done in the past, but that's problematic because we'd have to know which protocol is configured (http/https) and which port plus authentication etc.

Instead I've prototyped a JSONRestconfService interface and implementation that is advertised via the config system and can be consumed by apps. It contains the standard CRUD methods. The implementation accesses the front-end parsing code and RestconfImpl the same as external HTTP requests do. The service accepts and returns JSON but a similar service for XML could also be added.