sign method
Signs the given data
Implementation
@override
Future<Uint8List> sign(Uint8List data) async {
final signer = RSASigner(SHA256Digest(), '0609608648016503040201'); // SHA-256 with PKCS1v15 padding
signer.init(true, PrivateKeyParameter<RSAPrivateKey>(_key));
final signature = signer.generateSignature(data);
return signature.bytes;
}