addOTP method
This method is responsible for updating a card request with the card's OTP
Implementation
Future<ChargeResponse> addOTP(String otp, String flwRef) async {
return FlutterwaveAPIUtils.validatePayment(
otp,
flwRef,
http.Client(),
this.isDebugMode,
this.publicKey,
false,
MetricManager.VALIDATE_CARD_CHARGE);
}