Missing record types to use in record set

[lrolvink]

Currently there are 7 types defined as enum ("A", "AAAA", "NS", "CName", "MX", "SOA", "PTR"). Other common types like "CNAME", "TXT", "SRV" and "ALIAS" are missing. List of all known types can be found on https://doc.powerdns.com/authoritative/appendices/types.html

It will increase the usability of library if these types are added because also reading the zones also fails on this unfortunately.

Suggestion is to make the RecordType in RecordSet a string based value. Transform the RecordType into a string based list that contains all the known types.