+str #15079 Add flexible merge

Still only a prototype, but would be great with a round of feedback for tomorrow's work.

I tried the "return next behavior" approach (see savepoint1 if you are interested), but that was a dead end. We want to allow return of behavior with new input and output types, e.g. Zip, and I couldn't get that to work without explosion of types. Reverted back to become.

// cc @drewhk @sirthias 
For this to work with the GraphFlow there needs to be a way for the vertex to create a new "anonymized" instance of the concrete subclass of the FlexiMerge. That seems to be the only thing AFAICS, assuming that the new instance creates the InputPorts in the same order :wink:.