New selected_delivery_option field in cart

We have added the selected_delivery_option as part of the CartDeliveryGroup as well as the handle field as part of the CartDeliveryOption in the cart.

The selected_delivery_option field represents the selected delivery option in a delivery group. The handle field represents a unique identifier for the delivery option.

A default CartDeliveryOption is saved as the selected_delivery_option in each CartDeliveryGroup available.

We also added the cartSelectedDeliveryOptionsUpdate mutation, allowing you to update the selected_delivery_options of each of your delivery groups.