refactor: move base http3 transport into netxlite (#412)

This diff is part of https://github.com/ooni/probe/issues/1505.

You will notice that I have not adapted all the (great) tests we had
previously. They should live at another layer, and namely the one that
deals with performing measurements.

When I'm refactoring such a layer I'll ensure those tests that I have
not adapted here are reintroduced into the tree.

internal/netxlite/http3_test.go

@@ -0,0 +1,25 @@
+package netxlite
+
+import (
+	"crypto/tls"
+	"net"
+	"net/http"
+	"testing"
+)
+
+func TestHTTP3TransportWorks(t *testing.T) {
+	d := &QUICDialerResolver{
+		Dialer: &QUICDialerQUICGo{
+			QUICListener: &QUICListenerStdlib{},
+		},
+		Resolver: &net.Resolver{},
+	}
+	txp := NewHTTP3Transport(d, &tls.Config{})
+	client := &http.Client{Transport: txp}
+	resp, err := client.Get("https://www.google.com/robots.txt")
+	if err != nil {
+		t.Fatal(err)
+	}
+	resp.Body.Close()
+	txp.CloseIdleConnections()
+}