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/engine/netx/resolver/integration_test.go

@@ -64,7 +64,7 @@ func testresolverquickidna(t *testing.T, reso model.Resolver) {
 }
 
 func TestNewResolverSystem(t *testing.T) {
-	reso := &netxlite.ResolverSystem{}
+	reso := netxlite.NewResolverSystem()
 	testresolverquick(t, reso)
 	testresolverquickidna(t, reso)
 }