add: gateway custom response data

[seth-shi]

rpc response:

{
   "user": null
}

gateway add custom field to:

{
   "code": 200,
    "data": rpcData
}

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.30%. Comparing base (b22ad50) to head (904cb21).
Report is 385 commits behind head on master.

Additional details and impacted files

Files	Coverage Δ
gateway/internal/eventhandler.go	100.00% <100.00%> (ø)
gateway/server.go	69.42% <100.00%> (+0.51%)	⬆️

... and 2 files with indirect coverage changes

[lvphps]

ok