YANG Patch fromat printer for diff data

[rkrejci]

Provide data printer for diff data (lyd_diff_tree() / lyd_diff_siblings()) in YANG Patch format.

[rkrejci]

Just the printer will not be enough. We need full support for yang-data extension, which is the way how ynag-patch data are defined. Some initial implementation can also create the data as opaque nodes without any connection with the schema definition.