decryptConversation function
Implementation
Future<List<Message>> decryptConversation({
  required List<Message> messages,
  required User? connectedUser,
  required String pgpPrivateKey,
}) async {
  final updatedMessages = <Message>[];
  for (var msg in messages) {
    if (msg.encType != 'PlainText') {
      msg = await decryptAndVerifyMessage(
        pgpPrivateKey: pgpPrivateKey,
        pgpPublicKey: connectedUser!.publicKey!,
        message: msg,
      );
    }
    updatedMessages.add(msg);
  }
  return updatedMessages;
}