Add Placeholders for forward_proxy_url

[ImpostorKeanu]

Hi,

I worked with y'all to have the reverse_proxy.transport.forward_proxy_url field added to Caddy a few months ago and it's working just fine, but I failed apply placeholder values.

To make it more useful, I've branched and applied a patch here.

Happy to make a PR, but it seems like I made things more complicated than necessary last time around. Thoughts?

Here's how the change looks:

	// negotiate any HTTP/SOCKS proxy for the HTTP transport
	var proxy func(*http.Request) (*url.URL, error)
	if h.ForwardProxyURL != "" {
		logger := caddyCtx.Logger()
		proxy = func(r *http.Request) (*url.URL, error) {
			// retrieve the replacer from the context.
			repl, ok := r.Context().Value(caddy.ReplacerCtxKey).(*caddy.Replacer)
			if !ok {
				err := errors.New("failed to obtain replacer from request")
				logger.Error(err.Error())
				return nil, err
			}

			// apply placeholders to the value
			// note: h.ForwardProxyURL should never be empty at this point
			s := repl.ReplaceAll(h.ForwardProxyURL, "")
			if s == "" {
				logger.Error("forward_proxy_url was empty after applying placeholders",
					zap.String("initial_value", h.ForwardProxyURL),
					zap.String("final_value", s),
					zap.String("hint", "check for invalid placeholders"))
				return nil, errors.New("empty value for forward_proxy_url")
			} else if pUrl, err := url.Parse(s); err != nil {
				logger.Warn("failed to derive transport proxy from forward_proxy_url")
				return nil, err
			} else {
				logger.Debug("setting transport proxy url", zap.String("url", s))
				return pUrl, nil
			}
		}
	} else {
		proxy = http.ProxyFromEnvironment
	}

	rt := &http.Transport{
		Proxy:                  proxy,
		DialContext:            dialContext,
		MaxConnsPerHost:        h.MaxConnsPerHost,
		ResponseHeaderTimeout:  time.Duration(h.ResponseHeaderTimeout),
		ExpectContinueTimeout:  time.Duration(h.ExpectContinueTimeout),
		MaxResponseHeaderBytes: h.MaxResponseHeaderSize,
		WriteBufferSize:        h.WriteBufferSize,
		ReadBufferSize:         h.ReadBufferSize,
	}

[francislavoie]

A PR is certainly welcome, it's the easiest way to review and iterate on the change.

Makes sense to me, just one nitpick about the if chain - when each condition returns, there's no need for else, just do two ifs in a row and then the final case can be unwrapped from the else (and un-indented).