* feat: save CNAME into archival data format
When a DNSResponse contains a non-empty CNAME, we include it
into the related list of answers.
Closes https://github.com/ooni/probe/issues/2227
* doc: add design note
While there, make code more compact and robust to a case where
we're going to extract additional answers.
* doc: document the expected growth of extraction function
Based on feedback by @DecFox
60b7d1f87b