operator [] method
Implementation
@override
operator [](key) => switch (key) {
'to' => to?.key ?? '',
'hash' => hash,
'sig' => sig,
'pubkey' => pubkey,
'content' => content,
'owner' => owner?.hash,
'sync_at' => syncAt,
'created_at' => createdAt,
_ => super[key],
};