Add CNAME chasing for TXT records

[hslatman]

A potential fix for resolving TXT records that are behind CNAME records, as used by e.g. https://github.com/joohoi/acme-dns.

[hslatman]

User who reported the issue on Discord reported this fix doesn't work (yet). Unsure if it's a (local) DNS issue, or if something else is up. Am going to try to replicate this with an acme-dns setup.

[hslatman]

The issue turned out to be DNS 😝

When running step-ca in a container, with dnsmasq as a DNS server, the TXT query would result in a SERVFAIL if a CNAME had to be resolved. With bind9, there was no issue and the CNAME was resolved correctly to the TXT, without explicitly doing net.LookupCNAME. So, no changes required in how step-ca resolves queries now. Some additional diagnostics might help, though, for example in the form of a clearer error.

[Ziphone]

The drafted fix did the trick for me. I'm also using the ACME-DNS. As it appears to me, net.LookupTXT does not follow CNAME(s).