ZONEMD minimum digest length

[Philip-NLnetLabs]

RFC 8976, which defines ZONEMD, Section 2.2.4, requires the digest field to be at least 12 octets. Currently, Zonemd does not enforce this requirement.

[bal-e]

The requirement appears to be enforced when parsing from the wire format (https://docs.rs/domain/latest/src/domain/rdata/zonemd.rs.html#110-124) but not when parsing from a zonefile.