Finish CDS/CDNSKEY implementation

[vlevigneron]

• Specific methods for CDS/CDNSKEY ressource records need to be implemented.

Since the CDS (resp CDNSKEY) resource record format is identical to the DS record (resp DNSKEY record), we could make the Zonemaster::LDNS::RR::CDS (resp Zonemaster::LDNS::RR::CDNSKEY) a subclass of Zonemaster::LDNS::RR::DS (resp Zonemaster::LDNS::RR::DNSKEY).