WIP #16543 add directives to add or remove trailing slash

Added two directives as said in #16543.

Need to discuss:
- The location of the directives: `PathDirectives` or `RouteDirectives`?
- Returning status code to redirect: 301, 302 or make it configurable? 
- More tests
- More documentation
- etc.
Yes. That would also make the impl simpler although not tail recursive. See https://github.com/2beaucoup/akka/commit/a09bbe653d56001daa0d6033dcd26b3ff390047d.