refactor: DNSTransport I/Os DNS messages (#760)

This diff refactors the DNSTransport model to receive in input a DNSQuery and return in output a DNSResponse.

The design of DNSQuery and DNSResponse takes into account the use case of a transport using getaddrinfo, meaning that we don't need to serialize and deserialize messages when using getaddrinfo.

The current codebase does not use a getaddrinfo transport, but I wrote one such a transport in the Websteps Winter 2021 prototype (https://github.com/bassosimone/websteps-illustrated/).

The design conversation that lead to producing this diff is https://github.com/ooni/probe/issues/2099

internal/model/mocks/dnsencoder_test.go

@@ -5,24 +5,46 @@ import (
 	"testing"
 
 	"github.com/miekg/dns"
+	"github.com/ooni/probe-cli/v3/internal/model"
 )
 
 func TestDNSEncoder(t *testing.T) {
 	t.Run("Encode", func(t *testing.T) {
 		expected := errors.New("mocked error")
+		queryID := dns.Id()
 		e := &DNSEncoder{
-			MockEncode: func(domain string, qtype uint16, padding bool) ([]byte, uint16, error) {
-				return nil, 0, expected
+			MockEncode: func(domain string, qtype uint16, padding bool) model.DNSQuery {
+				return &DNSQuery{
+					MockDomain: func() string {
+						return dns.Fqdn(domain) // do what an implementation MUST do
+					},
+					MockType: func() uint16 {
+						return qtype
+					},
+					MockBytes: func() ([]byte, error) {
+						return nil, expected
+					},
+					MockID: func() uint16 {
+						return queryID
+					},
+				}
 			},
 		}
-		out, queryID, err := e.Encode("dns.google", dns.TypeA, true)
+		query := e.Encode("dns.google", dns.TypeA, true)
+		if query.Domain() != "dns.google." {
+			t.Fatal("invalid domain")
+		}
+		if query.Type() != dns.TypeA {
+			t.Fatal("invalid type")
+		}
+		out, err := query.Bytes()
 		if !errors.Is(err, expected) {
 			t.Fatal("unexpected err", err)
 		}
 		if out != nil {
 			t.Fatal("unexpected out")
 		}
-		if queryID != 0 {
+		if query.ID() != queryID {
 			t.Fatal("unexpected queryID")
 		}
 	})