fix: update the jsonrpc.Response struct to adhere to the JSONRPC spec

[commoddity]

Summary

Fixes the jsonrpc.Response struct used for qos to adhere to the JSON-RPC spec, as well as update its .Validate method.

Issue

There were some small discrepancies between the official JSON-RPC spec and our jsonrpc.Response struct.

Specifically the error object was always being included even without an actual error, which is prohibited by the JSON-RPC spec.

As well the presence of either neither or both of result and error was not being validated.

Source: https://www.jsonrpc.org/specification#response_object

5 Response object
When a rpc call is made, the Server MUST reply with a Response, except for in the case of Notifications. The Response is expressed as a single JSON Object, with the following members:

jsonrpc
A String specifying the version of the JSON-RPC protocol. MUST be exactly "2.0".

result
This member is REQUIRED on success.
This member MUST NOT exist if there was an error invoking the method.
The value of this member is determined by the method invoked on the Server.

error
This member is REQUIRED on error.
This member MUST NOT exist if there was no error triggered during invocation.
The value for this member MUST be an Object as defined in section 5.1.

...

Either the result member or error member MUST be included, but both members MUST NOT be included.

Type of change

Select one or more from the following:

• New feature, functionality or library
• Bug fix
• Code health or cleanup
• Documentation
• Other (specify)

Testing

• All Tests: make test_all
• Documentation: make docusaurus_start; only needed if you make doc changes

Sanity Checklist

• I have tested my changes using the available tooling
• I have commented my code
• I have performed a self-review of my own code; both comments & source code
• I create and reference any new tickets, if applicable
• I have left TODOs throughout the codebase, if applicable