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, but both of these should refer to same resource, no?
I guess we could add a `relativeRedirect` directive which does things right in the background but currently I'd say that giving redirects always an absolute target URI is actually a good thing.