enable utls for websteps (#442)

This diff enables `websteps` to use uTLS for TLS parroting. It integrates the `oohttp.StdlibTransport` wrapper which uses the `ooni/oohttp` fork. `oohttp` supports TLS-like connections like `utls.Conn`.
As a prototype, the testhelper and `websteps` code now uses the `utls.HelloChrome_Auto` fingerprint, i.e. the simulated TLS fingerprint of the Google Chrome browser.

It is a further contribution for my GSoC project.

Reference issue: https://github.com/ooni/probe/issues/1733

internal/engine/experiment/websteps/tls.go

@@ -1,16 +1,23 @@
 package websteps
 
 import (
+	"context"
 	"crypto/tls"
 	"net"
+
+	"github.com/ooni/probe-cli/v3/internal/netxlite"
+	utls "gitlab.com/yawning/utls.git"
 )
 
 // TLSDo performs the TLS check.
-func TLSDo(conn net.Conn, hostname string) (*tls.Conn, error) {
-	tlsConn := tls.Client(conn, &tls.Config{
+func TLSDo(ctx context.Context, conn net.Conn, hostname string) (net.Conn, error) {
+	tlsConf := &tls.Config{
 		ServerName: hostname,
 		NextProtos: []string{"h2", "http/1.1"},
-	})
-	err := tlsConn.Handshake()
+	}
+	h := &netxlite.TLSHandshakerConfigurable{
+		NewConn: netxlite.NewConnUTLS(&utls.HelloChrome_Auto),
+	}
+	tlsConn, _, err := h.Handshake(ctx, conn, tlsConf)
 	return tlsConn, err
 }