Initialize the Mobile Checkout SDK

After creating the configuration object, we can now initialize the checkout as follows:

CheckoutController.Instance.Init(callback);

CheckoutController.Instance.Init(customerId, callback);

Argument	Description
`callback`	A callback interface implementation: `ICheckoutPurchase`
`customerId`	Customer Identification string

The ICheckoutPurchase interface acts as a callback. The methods within the interface will be triggered based on the following events:

Method	Description	Arguments
`OnInitialized`	Triggered when the checkout initialization is completed.
`OnInitializeFailed`	Triggered when initialization has failed.	`ErrorMessage` containing a code and reason for the failure.
`OnPurchaseSuccess`	Triggered when the purchase is completed successfully.	`OrderResponseModel` containing detailed properties for the order.
`OnPurchaseFailed`	Triggered when the purchase fails for any reason.	`ErrorMessage` containing a code and reason for the failure.
`OnPurchaseConsumed`	Triggered when the purchase is consumed by the client.	`true` if the purchase is consumed successfully, or `false` otherwise.
`OnPricePointsSuccess`	Triggered when price points are successfully retrieved.	`PricePointsModel` containing details about the price points.
`OnPricePointsFail`	Triggered when price points are not retrieved properly.	`ErrorMessage` containing a code and reason for the failure.
`OnGetNonConsumeOrdersSuccess`	Triggered when none-consumed operation was success.	Message will contain a List of order IDs that are waiting for being consumed.
`OnGetNonConsumeOrdersFail`	Triggered when none-consumed operation failed to execute.	`ErrorMessage` containing a code and reason for the failure.