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/dnsquery_test.go

@@ -0,0 +1,62 @@
+package mocks
+
+import (
+	"bytes"
+	"testing"
+
+	"github.com/miekg/dns"
+)
+
+func TestDNSQuery(t *testing.T) {
+	t.Run("Domain", func(t *testing.T) {
+		expected := "dns.google."
+		q := &DNSQuery{
+			MockDomain: func() string {
+				return expected
+			},
+		}
+		if q.Domain() != expected {
+			t.Fatal("invalid domain")
+		}
+	})
+
+	t.Run("Type", func(t *testing.T) {
+		expected := dns.TypeAAAA
+		q := &DNSQuery{
+			MockType: func() uint16 {
+				return expected
+			},
+		}
+		if q.Type() != expected {
+			t.Fatal("invalid type")
+		}
+	})
+
+	t.Run("Bytes", func(t *testing.T) {
+		expected := []byte{0xde, 0xea, 0xad, 0xbe, 0xef}
+		q := &DNSQuery{
+			MockBytes: func() ([]byte, error) {
+				return expected, nil
+			},
+		}
+		out, err := q.Bytes()
+		if err != nil {
+			t.Fatal(err)
+		}
+		if !bytes.Equal(expected, out) {
+			t.Fatal("invalid bytes")
+		}
+	})
+
+	t.Run("ID", func(t *testing.T) {
+		expected := dns.Id()
+		q := &DNSQuery{
+			MockID: func() uint16 {
+				return expected
+			},
+		}
+		if q.ID() != expected {
+			t.Fatal("invalid id")
+		}
+	})
+}