=str #16163 allow connecting GraphSource to GraphSink directly

* introduce Identity junction, which is used to connect GraphSource to GraphSink
* only materialize ActorFlowSource and ActorFlowSink as a Flow

The solution is identical to a solution in a Flow materialization when if both Source and Sink are passive an identity transformer is used to connect the two.

Fixes #16163
Good point. Previous code had a fall-through to `runGraph()`.  I think we can change it to `IllegalStateException` so it would signal that graph preconditions have been fullfilled where it should not have.