Move payload into a sub attribute data in the request body

[stevehu]

This will make the schema validation works for the payload only and simply the schema definition.

After the change the typical json rpc request will look like this.

{
  "host": "lightapi.net",
  "service": "menu",
  "action": "createMenuItem",
  "version": "0.1.0",
  "data": {
    "menuItemId": "14",
    "label": "Access Admin",
    "route": "/admin/accessAdmin",
    "roles": [
      "admin",
      "owner"
    ]
  }
}