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.
Oops, missed that one. Question is: Why does it do that? It throws away the original request uri and replaces it with info that is also available through `effectiveUri`.

This makes relative redirection for relative request impossible AFAICS.