feat: parse PublicKey from the last valid label and not just the TLD.

[Nuhvi]

While I am very against using Pkarr keys relative to another TLD like .pkarr or .key or whatever, the idea of looking up pkarr keys from a DNS server by querying <key>.my.trusted.dns.server is very useful.

Maybe to support that and make it easier for everyone, we can define a Pkarr domain not as any domain that ends with a Pkarr TLD, but rather any domain that contains a Pkarr key as a label.

In case there are multiple keys in the domain like foo.<key B>.bar.<key A>.my.trusted.dns.server then <key A> is the parsed key and the one used in (for example) Endpoints resolution.