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.
I tried to come up with a name that communicates to condition was well as the target of the redirection and thought that "appended" and "stripped" would indicate both without getting too verbose. But I agree that the conditional aspect could get overlooked.

`redirectToTrailingSlashIfMissing`/`redirectToNoTrailingSlashIfPresent`?