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/netx.go

@@ -78,7 +78,7 @@ var defaultCertPool *x509.CertPool = netxlite.NewDefaultCertPool()
 // NewResolver creates a new resolver from the specified config
 func NewResolver(config Config) model.Resolver {
 	if config.BaseResolver == nil {
-		config.BaseResolver = &netxlite.ResolverSystem{}
+		config.BaseResolver = netxlite.NewResolverSystem()
 	}
 	var r model.Resolver = config.BaseResolver
 	r = &netxlite.AddressResolver{
@@ -260,7 +260,7 @@ func NewDNSClientWithOverrides(config Config, URL, hostOverride, SNIOverride,
 	}
 	switch resolverURL.Scheme {
 	case "system":
-		return &netxlite.ResolverSystem{}, nil
+		return netxlite.NewResolverSystem(), nil
 	case "https":
 		config.TLSConfig.NextProtos = []string{"h2", "http/1.1"}
 		httpClient := &http.Client{Transport: NewHTTPTransport(config)}