+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.
Sorry, of course not a Broadcast. But if the out1 and out2 this is like workers then where a real out?
I mean that router is a flow like:
```
       / w1--->---\
in -> w2----->---->-- > out
       \ w3---->---/
```
Of course you can specify a single out by merging all workers. But why you need doing this by hand?