[Feat] Rewrite r.kappa JSON output to use parson library

[petrasovaa]

Is your feature request related to a problem? Please describe.
JSON output was added to r.kappa before introducing parson library.

The idea is to rewrite the current implementation to use parson library
for better maintainability.

[Chirag3841]

@petrasovaa I would like to solve this issue if anyone had not taken it yet.