LastDeviceRecoveryKeyRegistration
class LastDeviceRecoveryKeyRegistration(val userId: UserId, publicKey: ByteArray, proofOfPossession: ByteArray)
A user's registration of publicKey as its last-device recovery key. proofOfPossession is the recovery key's signature over u32 length | domain | u32 length | userId | publicKey, so nobody can register a key they do not hold (a registration cannot be replaced in milestone 18).