Optimise - Allow to turn DataType on or off on Message

We can optimise to not create DataType instances on the Message instance if its not really needed.

Its part of the new route contract where it is used to know the data type and then do contract enforcement and validation/conversion if needed.

But for regular use-cases it has not so much value ... yet.

So we should maybe turn it off by default, and allow to turn it on|off, and then turn it on if you use contract enforcement.