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 still think this naming is ambiguous.
When talking about redirects here there is always the question:
Are we redirecting *to* a certain URI style or redirecting *from* it.
Ideally I'd like to very clear which of the two it is that the directive performs.

The problem is that the directive only directs a subset of the requests it gets and let's the others pass.
So `redirectToTrailingSlashIfRequired` and `redirectToNoTrailingSlashIfRequired` would actually be the most descriptive, albeit a bit long.