feat(netxlite): add dialer factory, simplify resolver factory (#459)

See https://github.com/ooni/probe/issues/1591

internal/netxlite/http3_test.go

@@ -13,7 +13,7 @@ func TestHTTP3TransportWorks(t *testing.T) {
 		Dialer: &quicDialerQUICGo{
 			QUICListener: &quicListenerStdlib{},
 		},
-		Resolver: NewResolver(&ResolverConfig{Logger: log.Log}),
+		Resolver: NewResolverSystem(log.Log),
 	}
 	txp := NewHTTP3Transport(d, &tls.Config{})
 	client := &http.Client{Transport: txp}