Add external payment methods

import StripePaymentSheet

class MyCheckoutVC: UIViewController {
    func setUpPaymentSheet() {
        // ...
        var configuration = PaymentSheet.Configuration()
        configuration.externalPaymentMethods = .init(
            externalPaymentMethods: ["external_fawry"]
        ) { [weak self] externalPaymentMethodType, billingDetails, completion in
            self?.handleExternalPaymentMethod(type: externalPaymentMethodType, billingDetails: billingDetails, completion: completion)
        }
        // ...
    }

    func handleExternalPaymentMethod(type: String, billingDetails: STPPaymentMethodBillingDetails, completion: @escaping (PaymentSheetResult) -> Void) {
        // Your implementation should complete the payment with the payment method provider
        // When the payment completes, cancels, or fails, call the `completion` handler
        // Note you can present on top of PaymentSheet by using the `self.presentedViewController`.
        // This example code just immediately fails:
        let exampleError = NSError(domain: "MyErrorDomain", code: 0, userInfo: [NSLocalizedDescriptionKey: "Failed to complete payment!"])
        completion(.failed(error: exampleError))
    }
}