+str #15205 Add Route junction

This is only an early draft so far.
It's symmetric to FlexiMerge and provides a similar API.

What have I not thought about?

Only happy case testing so far. I will add tests for cancel, error, completion etc.

I would be interesting to get feedback from @sirthias to see if this together with FlexiMerge solves feature request of a [more general `zip`](https://github.com/akka/akka/issues/15341), or what is missing.
Your use case is interesting, but unrelated to this pull request.
The `StrictRoundRobin` is just a sample/test of the new `Route` feature.
What you describe looks similar to what is illustrated here: https://github.com/akka/akka/blob/release-2.3-dev/akka-stream-tests/src/test/scala/akka/stream/scaladsl/GraphOpsIntegrationSpec.scala#L15
Such building blocks can be provided on top of the more general operators.