getPrivateKey method
Get a private key for a specific change and address index, throwing an exception if it's a public-only key.
Implementation
Secp256k1PrivateKey getPrivateKey(int changeIndex, int addrIndex) {
  if (isPublicOnly) {
    throw const MessageException(
        'Public-only deterministic keys have no private half');
  }
  return _derivePrivateKey(changeIndex, addrIndex);
}