Option of product in the cart.

Column	Type	Constraint	Description
shop	TEXT	PRIMARY KEY	Shop ID. (shop.username)
cart_id	INTEGER		Cart ID. (cart.id)
index	INTEGER		Index of product in the cart. (cart_product.index)
option_id	INTEGER		Option ID. (option.id)
quantity	INTEGER	NOT NULL	Quantity.

CREATE TABLE cart_product_option(
shop       TEXT,
cart_id    INTEGER,
index      INTEGER,
option_id  INTEGER,
quantity   INTEGER NOT NULL,
PRIMARY KEY(shop, cart_id, index, option_id)
);