+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
Or maybe if `UndefinedSource` was `UndefinedInput, and `UndefinedSink` was `UndefinedOutput`? Oh wait bikeshedding again...