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.go

@@ -0,0 +1,33 @@
+package mocks
+
+//
+// Mocks for model.DNSQuery.
+//
+
+import "github.com/ooni/probe-cli/v3/internal/model"
+
+// DNSQuery allocks mocking model.DNSQuery.
+type DNSQuery struct {
+	MockDomain func() string
+	MockType   func() uint16
+	MockBytes  func() ([]byte, error)
+	MockID     func() uint16
+}
+
+func (q *DNSQuery) Domain() string {
+	return q.MockDomain()
+}
+
+func (q *DNSQuery) Type() uint16 {
+	return q.MockType()
+}
+
+func (q *DNSQuery) Bytes() ([]byte, error) {
+	return q.MockBytes()
+}
+
+func (q *DNSQuery) ID() uint16 {
+	return q.MockID()
+}
+
+var _ model.DNSQuery = &DNSQuery{}