Create order session

This is the initial point in the lifecycle of an order. A successful request response will contain a sessionId and an orderId string. The sessionId will be needed later for rendering the checkout iframe, fetching and updating the session etc.

The session will expire after 7 days.

When the end customer has succesfully completed the checkout flow and the order session is in state authorized, the system will yield an order entity managed by the /orders endpoint. The orderId can now be used for Order management.