addInstrument method
adds new instrument to the customer for future use
Implementation
Future<void> addInstrument({required CreditCard creditCard}) async {
final customer = state.value;
final token = await _requestToken(creditCard);
final instrument = await _createInstrument(token, customer!);
state = AsyncValue.data(
customer.copyWith(
instruments: [...customer.instruments, instrument],
defaultInstrument: customer.instruments.isEmpty ? instrument : null),
);
}