feat(dns): expose raw response message

This diff extends measurexlite's extractor to include into the
DNS response we submit the raw response we received.

The reference issue is ooni/probe#1516

internal/measurexlite/dns.go

@@ -119,12 +119,21 @@ func NewArchivalDNSLookupResultFromRoundTrip(index int64, started time.Duration,
 		Failure:          tracex.NewFailure(err),
 		Hostname:         query.Domain(),
 		QueryType:        dns.TypeToString[query.Type()],
+		RawResponse:      maybeRawResponse(response),
 		ResolverHostname: nil,
 		ResolverAddress:  reso.Address(),
 		T:                finished.Seconds(),
 	}
 }
 
+// maybeRawResponse returns either the raw response (when available) or nil.
+func maybeRawResponse(resp model.DNSResponse) (out []byte) {
+	if resp != nil {
+		out = resp.Bytes()
+	}
+	return
+}
+
 // archivalAnswersFromAddrs generates model.ArchivalDNSAnswer from an array of addresses
 func archivalAnswersFromAddrs(addrs []string) (out []model.ArchivalDNSAnswer) {
 	for _, addr := range addrs {

internal/model/archival.go

@@ -117,6 +117,7 @@ type ArchivalDNSLookupResult struct {
 	Failure          *string             `json:"failure"`
 	Hostname         string              `json:"hostname"`
 	QueryType        string              `json:"query_type"`
+	RawResponse      []byte              `json:"raw_response"`
 	ResolverHostname *string             `json:"resolver_hostname"`
 	ResolverPort     *string             `json:"resolver_port"`
 	ResolverAddress  string              `json:"resolver_address"`