Generate structures for RFC8040 yang-data extensions

RFC8040 has defined a new language extension, yang-data, which allows defining free-standing structures in YANG.

Add a mapping of these structures to Java, similar to other YANG constructs. The result should be similar to a top-level container, except it should not use 'ChildOf', but rather be a free-standing.

In terms of namespace collision handling, this construct should be secondary only to schema tree statements, i.e. in case of a conflict with grouping/identity, those should be displaced. A reasonable suffix needs to be assigned as well.

Note that these extensions are superseded with more flexible RFC7891 Yang Structure extensions – and when we implement them, they should map to the same Java construct.