Fix: JSON deserialiser

[punit-kulal]

Bug: json message isn't correctly serialised, even when published in protobuf compliant JSON format.

Context:

1. Protobuf style guide states that JSON keys should be in camelCase, and protobuf keys/field names should be in snake_case.
2. Thus, when a Request Payload is serialised. It uses CamelCase.
3. However, since raccoon uses standard encoding/json package to deserialise, it does not correctly deserialise camelCase keys of Json.

Fix:

1. Start using protobuf's official encoding/protojson package for deserialisation.
2. It adhere's to the style guide of protobuf, which supports deserialisation of both snake_case and camelCase keys in JSON.

[punit-kulal]

Closing due to large scope and backward incompatibility. Refer