net/url: add URL.RawFragment, URL.EscapedFragment

net/url: add URL.RawFragment, URL.EscapedFragment

These are analogous to URL.RawPath and URL.EscapedPath
and allow users fine-grained control over how the fragment
section of the URL is escaped. Some tools care about / vs %2f,
same problem as in paths.

Fixes #37776.

Change-Id: Ie6f556d86bdff750c47fe65398cbafd834152b47
Reviewed-on: https://go-review.googlesource.com/c/go/+/227645
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>