Dns_server.AuthenticationSourceAuthentication, stored in a Dns_trie with privileges to operations embedded in the name.
A key is a pair of a `raw Domain_name.t and a Dnskey.t. In the name, operation privileges and potentially IP addresses are encoded, e.g. foo._transfer.example.com may do AXFR on example.com and any subdomain, e.g. foo.example.com.
The type of operations, sorted by highest ot lowest privileges, an `Update may as well carry out a `Transfer.
operation_to_string op is the string representation of op.
access_granted ~required key_operation is true if key_operation is authorised for required operation.
zone_and_operation key is Some (zone, op), the zone of the key, and its operation op. If the key is not in the expected format, None is returned.
access op ~key ~zone checks whether key is authorised for op on zone.
Opaque type for storing authentication keys.