+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.
Maybe this `Fair` route should showcase that you can wait for demand from all downstream as the initial state to make it more fair. If not in this test, then at least in the docs in the future.