[Platform][API] Nodeinstance, kmsconfig Raw Json Cleanup

[mahendranbhat]

Description :
There are places where we just create raw json node and return that. Ebeans already has built in serialization and deserialization for DBJson fields.
In many cases these entities are also part of out API request responses and they end up polluting the API definition with raw opaque json fields instead of first class objects. Hence making this part of our API cleanup.
As a side effect this leads to much better code redability.

Fields included:

NodeInstanceJson
kmsconfig

[mahendranbhat]

https://phabricator.dev.yugabyte.com/D11935