Implement json path updates for xflatten

[Khatskevich]

since we have nullable records, unions, maps, and arrays, one may want to update just some subelement of those complex types, however, it is impossible without json path.

The feature need to have two problems to be solved before it is implemented:

• json path updates in Tarantool (currently implementing)
• Some api changes. For now it is impossible to distinguish cases
  1. update subelement
  2. create subelement and its parent
  3. replace subelement and its parent