Support RFC 8791 (YANG Data Structure Extensions)

[kwatsen]

Enable Yangson to validate instance documents that have been modeled using the sx:structure extension statement. Likely SHOULD also support the sx:augment-structure extension.

Bonus: also support RFC 8040's rc:yang-data extension statement. This may seem no longer important, keep in mind that the RESTCONF protocol is very important and accessing its yang-errors and yang-api models is something I do often, albeit after hacking the YANG module to expose the inner-container as a top-level config false node...

[kwatsen]

This is needed if desired to use Yangson to transcode errors encoded in XML.