!str #16400 Add Java API for StreamTcp

* move io.StreamTcp to scaladsl.StreamTcp
* add javadsl.StreamTcp that delegates to scaladsl implementation
* move impl
* add java test
Merge pull request #16550 from akka/wip-16400-javadsl-stream-io-patriknw

!str #16400 Add Java API for StreamTcp