Add DNS registrar

The DNS registration option is already implemented on the station side and there is a client version in gotapdance.

This implementation mostly matches the gotapdance cli's DNS registration option and gives the same result for DoH:

[18:21:56] [0-ff1a45] Representative - 5694a03dbcfdef404bcfa9f0d3769308573abfd0095145c58f665abb0efe1f82
[18:21:56] DNS payload length: 100
2025/05/15 22:21:56 send: name is longer than 255 octets (try using a shorter base domain?), current length: 265

So at least some additional work will be required to get this fully working.