Merge kasp.Key functionality into zone.FileZoneKey

Merge kasp.Key functionality into zone.FileZoneKey

Make zone.FileZoneKey the single representation of a file-backed key
(typically generated by dnssec-keygen). Move the common key-related
functionality into zone.FileZoneKey, and extend that functionality in
kasp.Key to also add state and timing related operations on top. Remove
duplicate into_ta() function.

Note that is_ksk() is implemented differently for kasp.Key: with the
metadata file available, the KSK status is loaded from that file, as it
indicates the authoritative policy decision which makes the key a KSK.
In zone.FileZoneKey which doesn't work with the metadata file, the KSK
status if inferred from the DNSKEY SEP flag - the best information
available for that class.

Assisted-by: Claude:claude-opus-4-8