(PE-5859) Keep entire request URI matching regex

When wrap-proxy is called with a regex, keep the entire
portion of the request URI matching the regex rather than
replacing it in the rewritten URI.
We definitely don't want to add a Jetty dependency here.  Adding an Apache one would be fine, that's likely to happen sooner or later with any library that's doing web stuff.  Personally I'd be OK with creating a separate ticket for that and not blocking this PR on it, though, if @camlow325 et al are cool with that.