When creating a charge, recurring or subscription session it is now possible to define a
payment_method_reference parameter. The payment method reference is stored on saved payment methods (e.g. card) if the session results in a saved payment method.
The payment method reference is returned in the
customer_payment_method_added webhook, allowing to link the webhook to a specific session/order.
payment_method_reference is not defined, the session id will be used as default.