+str,htc #16071, #16072 New Stream Tcp and Http API

* StreamTcp and Http extensions now return Flows and Sources that can be materialized later
* Flow can now be completed with another flow to be turned into a runnable flow
* Java side of things is missing so tests will fail
Not really since a `SinkSource` AKA `Terminator` AKA `FlowWithMagicHappening` is a `Sink` even when it is connected at both ends, maybe even in a circle.