Cart: slim item model

[adedw]

List items should be slim:

{
  "items": [
    {
      "item": {
        "uuid": "3fa85f64-5717-4562-b3fc-2c963f66afa6",
        "name": "string",
        "price": {
          "value": 0,
          "currency": "string"
        }
      },
      "quantity": 0
    }
  ],
  "total": {
    "value": 0,
    "currency": "string"
  }
}