Sign the items in the cart with a GPG key.

        After everything is collected and signed all the cart items
        are issued a refresh() to sync their is_signed attributes.

        `sign_with` is a reference to the method that implements
        juicer.common.RpmSignPlugin.

def sign_items(self, sign_with):
        """
        Sign the items in the cart with a GPG key.

        After everything is collected and signed all the cart items
        are issued a refresh() to sync their is_signed attributes.

        `sign_with` is a reference to the method that implements
        juicer.common.RpmSignPlugin.
        """
        cart_items = self.items()
        item_paths = [item.path for item in cart_items]
        sign_with(item_paths)

        for item in cart_items:
            item.refresh()