Add support for TLS WebSocket proxy

[t-ibayashi-safie]

This PR fixes the issue below.
#2200

Detail

• middleware/proxy.go
  • I change proxyRaw method to be able to proxy tls connection
• middleware/proxy_test.go
  • I added two tests. In this test, I use golang.org/x/net/websocket

About websocket package in testing

I used an external package to easily implement web socket testing.

We considered the following packages, but we thought it would be better not to add too many third-party package dependencies for echo, so we adopted the official golang.org/x/net/websocket.

https://pkg.go.dev/github.com/gorilla/websocket
https://pkg.go.dev/github.com/coder/websocket

[aldas]

Maybe this would be better approach

func proxyRaw(t *ProxyTarget, c echo.Context, config ProxyConfig) http.Handler {
	var dialFunc func(ctx context.Context, network, addr string) (net.Conn, error)
	if transport, ok := config.Transport.(*http.Transport); ok {
		if transport.TLSClientConfig != nil {
			d := tls.Dialer{
				Config: transport.TLSClientConfig,
			}
			dialFunc = d.DialContext
		}
	}
	if dialFunc == nil {
		var d net.Dialer
		dialFunc = d.DialContext
	}

	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		in, _, err := c.Response().Hijack()
		if err != nil {
			c.Set("_error", fmt.Errorf("proxy raw, hijack error=%w, url=%s", err, t.URL))
			return
		}
		defer in.Close()
		out, err := dialFunc(c.Request().Context(), "tcp", t.URL.Host)
		if err != nil {
			c.Set("_error", echo.NewHTTPError(http.StatusBadGateway, fmt.Sprintf("proxy raw, dial error=%v, url=%s", err, t.URL)))
			return
		}

[aldas]

LGTM!

p.s. huge thanks for the raw proxy tests! I have never found motivation to add these myself and always felt bad when reviewing proxy related PRs for that.