net/http: implement path value methods on Request

net/http: implement path value methods on Request

Add Request.PathValue and Request.SetPathValue,
and the fields on Request required to support them.

Populate those fields in ServeMux.ServeHTTP.

Updates #61410.

Change-Id: Ic88cb865b0d865a30d3b35ece8e0382c58ef67d1
Reviewed-on: https://go-review.googlesource.com/c/go/+/528355
Run-TryBot: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>