Cut generators in CPModelProto

Hi

Following on from #666 - I found the reason why I wasn't using the `CPModelProto`. I've got some custom cutting planes I'm implementing over a SAT model and using the CutGenerator callbacks.

I wasn't able to find an interface on the `CPModelProto` that allows for adding a cut-callback. I would prefer to work at the CPModel level rather than SAT (as per Laurents' suggestion) but perhaps I'm just missing the part of the API that gives the appropriate functionality?

It could be that it's not available at that level - passing a pointer to a callback over the proto model seems like a bad idea and something that probably wouldn't be supported. I suppose this issue is simply then for confirmation that this is the case or not.

Thanks