signTransaction method
Signs a transaction using the account's private key.
Implementation
@override
SuiSecp256r1Signature signTransaction(List<int> txbytes,
    {bool hashDigest = true}) {
  final digest = SuiCryptoUtils.generateTransactionDigest(
      txBytes: txbytes, hashDigest: hashDigest);
  final signature = privateKey.sign(digest);
  return SuiSecp256r1Signature(
      publicKey: publicKey.publicKey, signature: signature);
}