Add pattern syntax for optional trailing slash

[tp971]

I proposed this idea in https://github.com/actix/actix-web/discussions/3258#discussioncomment-8358174 to handle cases where trailing slashes are relevant. Basically, allow defining a route with a pattern like "/some/path/?" that matches both "/some/path" and "/some/path/". I also proposed to add a method to check whether the URI has a trailing slash, but that can also be done with request.uri().path().ends_with('/'), so idk whether we actually need this.

[deepto98]

@robjtede I'd like to work on this, could you point me to the files I need to look at?

[robjtede]

@deepto98 mostly actix-router's ResourceDef... it's very well documented but lmk here or in Discord if you have questions