+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.
Possible I will be wrong or missed anything, but from the user point of view: Route ~ Router and I was confused when saw this code.
I was thinking it will be like:
```
new StrictRoundRobin[String](10/* amount */, in ~> map ~> ... ~> out /* routee */)
```

Currently it seems like the Broadcast instead Route.