Support for printing and parsing bare json values for sysrepo-gnmi

[irfanHaslanded]

• Support for parsing JSON bare leaf values

  Add support for parsing bare values for top-level (from the parser's point of view) leaf nodes, using a new LYD_OPT_BARETOPLEAF flag. The parsing of leaf nodes is too tied in with the allocation of a new node and parsing of the node name and namespace to easily trigger it from that case, so instead the case is check and handled explicitly in the top-level parser loop.

• Support for printing JSON bare values

  Add support for printing top-level leaf nodes as bare values, which is triggered through the use of a new LYD_BARETOPLEAF flag.

[jktjkt]

Neither the commit message nor the PR description explains why this is needed. How is it related to YANG or any of its serialization formats? What is the use case, i.e., the problem that this PR solves for you? What you're doing and why you need that?

Also, the parsing code has no documentation, and the one added test works with a simple string. IMHO that doesn't really show why this is needed.