getPriorityFee method
Implementation
Future<BigInt> getPriorityFee() async {
  // Direct RPC call to get suggested priority fee
  final response = await _call<String>(
    'eth_maxPriorityFeePerGas',
  );
  final priorityFee = response.toBigIntOrNull;
  if (priorityFee == null) throw Exception('Could not parse priority fee');
  return priorityFee;
}