=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
Hm. We should be able to do this without injecting NOP nodes... I guess we need to rethink the internal graph representation at some point.