Add simple support for CNAMEs as DNS aliases

[ryansouza]

For testing platform integrations with an ACME provider it would be nice to have simple but real-ish CNAMEs. This would allow testing the 2-part challenge setup where a customer CNAMEs their _acme-challenge fqdn to a platform-controlled fqdn where the actual TXT record is set up. This change will also respond to CNAME requests, allowing a mocked "has the customer CNAME'd their domain" check to run.

Planned implementation of this in the pebble-challtestsrv: https://github.com/letsencrypt/pebble/compare/master...ryansouza:cnames which I used to test this locally

[ryansouza]

@cpu thanks for the review 👍

I've changed the solution a bit to better match a real DNS server, and simplify the code changes a bit. Now a request for a CNAME'd host gets the CNAME record in addition to the requested records on the target host. I think it's a lot better than each individual lookup worrying about a CNAME

Heres an example of using it with the in-prog pebble-challtestsrv changes

$ curl localhost:8055/set-cname -XPOST --data '{"host":"foo.example.com", "target":"foobar.example.com"}'

$ curl localhost:8055/set-txt -XPOST --data '{"host":"foobar.example.com.", "value":"foobar"}'

$ curl localhost:8055/add-a -XPOST --data '{"host":"foobar.example.com", "addresses":["1.2.3.4"]}'

$ dig @localhost -p 8053 foo.example.com +noall +answer
foo.example.com.    0   IN  CNAME   foobar.example.com.
foobar.example.com. 0   IN  A   1.2.3.4

$ dig TXT @localhost -p 8053 foo.example.com +noall +answer
foo.example.com.    0   IN  CNAME   foobar.example.com.
foobar.example.com. 0   IN  TXT "foobar"

[cpu]

I cut a minor version release with this feature: https://github.com/letsencrypt/challtestsrv/releases/tag/v1.1.0 That should let you update the go.mod for the Pebble side of this work. Thanks again!