DNS wildcard support

[emmaly]

I don't believe we currently support *.example.com records, though we should.

Reference:

• https://tools.ietf.org/html/rfc1034#section-4.3.3
• https://en.wikipedia.org/wiki/Wildcard_DNS_record

Support for this is optional, though recommended. If we stop relying on an upstream resolver and do our own full-chain resolution, we are required to support them per spec. Until then, upstream does it for us. Upstream doesn't, though, deal with netcore-hosted wildcards, of course.