Milestone: Better anyxml support in yang-data and codecs

anyxml is used to model various data, but one particular common use-case
is that anyxml is actually used for YANG modeled data, which were inlined
in RPC or Notification and originated in Config / Operational Data Tree.

With correct extensions designed and supported yang-data-impl, XML and JSON
codec could use that to better understand data and provide better utilities
to work with anyxml data.