auth: Implement RFC 7872 and 9018 (COOKIES)

auth: Implement RFC 7872 and 9018 (COOKIES)

This implements the siphash-based interoperable DNS COOKIES defined in
RFC 9018 for the authoritative server. The EDNSCookieOpt struct has been
expanded to accomodate this and can now has constructors and functions
to check and generate a server cookie.

Cookies will only be sent out if the client sent a cookie and the
edns-cookie-secret setting is configures. The auth will respond with
EDNS+FORMERR when the client cookie is malformed, BADCOOKIE when the
client sent a server cookie we can't decode or is invalid and a normal
response with a cookie (either new or sent by the client) when the
cookie can be validated.