Pay by link API integration

This integration is to use April's backend API to create and send the order to the user. It then needs to detect when a payment has completed.

1. Initiating system must create an order in the April API by making an API call from your backend server with your merchant secret token or marketplace API key to Create Order using the CreatePayByLink action. This will take the user mobile number and/or email address and will also take the order details (total amount & line items).
2. Invoice and payment options will be sent to the user Mobile phone and/or email for verification.
3. If the initiating system wishes to be notified of the outcome of the order, they have two options:
   1. Poll - the system can poll the April order status API using Get Order , waiting on payment confirmation or failure. It is recommended that the poll method have a reasonable timeout period at which point the point of sale system should terminate the polling request and cancel the order via a Cancel Order API call.
   2. Register a webhook subscription - webhooks let you register an HTTP endpoint April will call whenever events of a particular type occur for your merchant or marketplace. By registering for order events you can be notified if and when an order is paid for. As with the polling method, you may also want to automatically cancel orders that are not completed within a reasonable time.