* Adds custom dial func for HTTP-Connect and socks5 proxy tunnels.
Standard golang http.transport exposes GetProxyConnectHeader function,
but it doesn't allow to use separate tls config for proxy.
It also not possible to enforce HTTP-Connect with standard http lib.
* For http scrape targets, by default http.Transport.Proxy function must
be used. Since it has special case with full uri forward.
* Adds proxy.URL json methods that allow to properly copy internal
fields, like User/Password.
It should fix bug with proxy_url. When credentials specified at URL was
ignored.
* Adds tests for scrape client proxy requests
related issue https://github.com/VictoriaMetrics/VictoriaMetrics/issues/6771
