Add support for RFC8791 YANG Data Structure Extensions

RFC7950 models RPCs, Notifications and datastore content, while RFC8040 defines a limited, reusable yang-data template.

RFC8791 generalizes RFC8040 to provide the construct for an externally-augmentable piece of data.

Add support for the YANG extensions defined in RFC8791 – both base structure and its augment. This may get a bit messy downstream, as suddenly data-api augmentations are served by two model-api statements – each of which has different semantics.

Also note that the parser-support part of this task needs to mirror AugmentStatementSupport  mechanics, so may end up being quite contrived.