Gets keys for named crypt filters that do not use the file encryption key, such as a public-key handler's crypt filters with
their own recipients (§7.6.6, Table 27); null when every crypt filter uses the file encryption key. An empty
key marks a crypt filter the credentials are not authorized for: streams that name it stay encrypted (§7.6.6).
public IReadOnlyDictionary<CosName, ReadOnlyMemory<byte>>? CryptFilterKeys { get; init; }