prepareRenewAsync 
suspend fun prepareRenewAsync(privateKeys: PreGeneratedKeys? = null): ByteArray
Prepare a private key renewal, so it can be stored on SSKS without risk of loss during the actual renew
Parameters 
| privateKeys | Optional. Pre-generated private keys, returned by a call to generatePrivateKeys or generatePrivateKeysAsync. | 
Throws 
| SealdException |