created common payment system document for MVC

Author: ismcagdas

docs/en/Features-Mvc-Core-Common-Payment-System.md

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+# Common Payment System
+
+ASP.NET Zero provides a payment system to get payments easily. In order to start a payment, just use `CreatePayment` method of `IPaymentManager` and then redirect user to `GatewaySelection` action of `PaymentController` with the created paymentId. For example, a sample URL for the `paymentId=1` should be like this;
+
+```bash
+https://localhost:44302/Payment/GatewaySelection?paymentId=1
+```
+
+ASP.NET Zero's common payment system will handle the rest of the payment flow. 
+
+- If the payment process is successfull, user will be redirected to `SuccessUrl` provided when creating the payment request.
+- If the payment process is unsuccessfull, user will be redirected to `ErrorUrl` provided when creating the payment request.
+- If `SuccessUrl` or `ErrorUrl` is not provided, user will be redirected to a common page and result of the payment process will be displayed.
+
+## Creating Payment Request
+
+### SubscriptionPayment Entity
+
+`IPaymentManager` is used to create a payment requet. It's `CreatePayment` method requires a `SubscriptionPayment` entity. Here is the detials of `SubscriptionPayment` entity.
+
+* `TenantId`: Represents which Tenant this payment request belongs to.
+* `PaymentPeriodType`: Period type of the payment if this is a payment for a specific period. Currently, Monthly and Annual are supported.
+* `DayCount`: Integer value of `PaymentPeriodType` field.
+* `Gateway`: Name of payment gateway which processed this payment. This is set by ASP.NET Zero when the payment is successfull.
+* `Status`: Status of payment. This is set by ASP.NET Zero. 
+* `ExternalPaymentId`: Id of the payment in the external payment gateway system like Stripe or PayPal. This is set by ASP.NET Zero. 
+* `InvoiceNo`: Invoice number if an invoice generated in ASP.NET Zero for this payment.
+* `SuccessUrl`: URL to redirect user if payment is successfull.
+* `ErrorUrl`: URL to redirect user if payment is failed.
+* `IsRecurring`: Represents if this is a recurring payment or not. If it is recurring, user's credit card will be charged at the end of every payment cycle. This is only supported by Stripe at the moment.
+* `IsProrationPayment`: This is a special field. If the tenant is on a recurring payment plan and operation is upgrade, then it is a proration payment.
+* `ExtraProperties`: A dictionary to store additional information on the payment object. 
+* `SubscriptionPaymentProducts`: List of products to be purchased for this payment. 
+
+### SubscriptionPaymentProduct Entity
+
+* `SubscriptionPaymentId`: Id of the related payment record.
+* `Description`: Description of the purchased product.
+* `Amount`: Price of the product.
+* `Count`: Count of products to be purchased.
+* `TotalAmount`: Total price of products to be purchased.
+* `ExtraProperties`: A dictionary to store additional information on the product object.