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/dnsresponse.go

@@ -0,0 +1,47 @@
+package mocks
+
+//
+// Mocks for model.DNSResponse
+//
+
+import (
+	"net"
+
+	"github.com/ooni/probe-cli/v3/internal/model"
+)
+
+// DNSResponse allows mocking model.DNSResponse.
+type DNSResponse struct {
+	MockQuery            func() model.DNSQuery
+	MockBytes            func() []byte
+	MockRcode            func() int
+	MockDecodeHTTPS      func() (*model.HTTPSSvc, error)
+	MockDecodeLookupHost func() ([]string, error)
+	MockDecodeNS         func() ([]*net.NS, error)
+}
+
+var _ model.DNSResponse = &DNSResponse{}
+
+func (r *DNSResponse) Query() model.DNSQuery {
+	return r.MockQuery()
+}
+
+func (r *DNSResponse) Bytes() []byte {
+	return r.MockBytes()
+}
+
+func (r *DNSResponse) Rcode() int {
+	return r.MockRcode()
+}
+
+func (r *DNSResponse) DecodeHTTPS() (*model.HTTPSSvc, error) {
+	return r.MockDecodeHTTPS()
+}
+
+func (r *DNSResponse) DecodeLookupHost() ([]string, error) {
+	return r.MockDecodeLookupHost()
+}
+
+func (r *DNSResponse) DecodeNS() ([]*net.NS, error) {
+	return r.MockDecodeNS()
+}