hproxy: use JSON-RPC responses

[AL-CT]

Confirmation

• I have checked for similar issues.

Problem

Hproxy should write responses in JSON-RPC format, rather than basic HTTP responses. This will allow for clearer errors and information being relayed to the client, as HTTP Status codes are currently 'abused' or over-generalized to fit the use-case.

Suggested solution

Replace the current code to respond to requests with JSON-RPC responses and errors.

Additional Information

No response