remove func_returns property

hey @daniel-eggert! As you know, the requests currently define a func_returns property from where you get the return value. This is a bit stupid as you always send the request back that you get, plus this additional property. I implemented it in the past like that to be able to include the return value in the generated JSONschema. However, meanwhile I think that does not make sense...

So what about removing this property and let the payload be the return value of the function? And if one is interested in the SCHEMA, I implement another MessageType? Such as

class MessageType(str, Enum):
    ...
    INFO = "info"
    RETURNINFO = "return-info"
    ...