refactor(netxlite): introduce the getaddrinfo transport (#775)

This diff modifies the system resolver to use a getaddrinf transport.

Obviously the transport is a fake, but its existence will allow us
to observe DNS events more naturally.

A lookup using the system resolver would be a ANY lookup that will
contain all the resolved IP addresses into the same response.

This change was also part of websteps-illustrated, albeit the way in
which I did it there was less clean than what we have here.

Ref issue: https://github.com/ooni/probe/issues/2096

internal/cmd/oohelperd/internal/webconnectivity/webconnectivity_test.go

@@ -53,7 +53,7 @@ func TestWorkingAsIntended(t *testing.T) {
 		Client:            http.DefaultClient,
 		Dialer:            netxlite.DefaultDialer,
 		MaxAcceptableBody: 1 << 24,
-		Resolver:          &netxlite.ResolverSystem{},
+		Resolver:          netxlite.NewResolverSystem(),
 	}
 	srv := httptest.NewServer(handler)
 	defer srv.Close()