Purchase multiple quantity of one product

[vlad0209]

In my pull request I added ability to purchase multiple quantity of one product using the in_app_purchase plugin. This is going to work only with the App Store underlying store. You can define a quantity in AppStorePurchaseParam. Here is example of using new feature:
purchaseParam = AppStorePurchaseParam( productDetails: productDetails, quantity: 5, applicationUserName: null, ); _inAppPurchase.buyConsumable(purchaseParam: purchaseParam);