Add validation for the message body in middleware worker

[Relrin]

As you can see in the appopriate documentation for a worker that works with the middleware queue, the enduser must specify the game-mode field in request of String type, however, the worker isn't applying any checks / validation rules for the message body.

So, as the enduser, I'm expecting that I will receive a validation error for the invalid message content when the required fields aren't specified, missing or have invalid types. Otherwise the message must be delivered to the next processing stage without any errors.