=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
are we sure that it can only be SourceVertex/SinkVertex here?
perhaps change the `case _` to `case (sourceVertex: SourceVertex, sinkVertex: SinkVertex)` and have a default `_` throwing `IllegalArgumentException`?