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.
Relative redirects would be I use case I can think of right now, where the first one would be redirected to `/redirectedFoo` and the second one to `http://example.com/bar/../redirectedFoo`.