Allows an app dev to generate extra fields in the in-memory backend. This enables the in-memory drivers to return responses similar to what the frontend would expect from the production platform. The value returned by the hook function will be set to the returned cart's `extra_attributes` field for driver calls that return a cart. The following example demonstrates adding the `numberOfCartItems` field to `extra_attributes`: ```ts (reqInfo: RequestInfo, cart: DaffCart) => ({ numberOfCartItems: cart.items.length }) ``` Note that this and any `extra_attributes` features are for advanced users and should be used with care.