Creates a CartItem resource.

curl -X POST "/api/carts/{cartId}/cart_items" \
  -H "apikey: $MEDIALISTER_API_KEY" \
  -H "Content-Type: application/ld+json" \
  -d '{
  "product": "/api/formats/0a097745-ff39-4659-8cae-a9bfb596a124",
  "campaign": "/api/campaigns/abcd32bd-a699-4f3e-bb19-ea807a0a6fc8",
  "qty": 1
}'

{
  "@context": "string",
  "@id": "string",
  "@type": "string",
  "items": [
    {
      "@context": "string",
      "@id": "string",
      "@type": "string",
      "product": {
        "@context": "string",
        "@id": "string",
        "@type": "string",
        "mediaProject": "/api/media_projects/b64ed0ca-1a05-4898-9a52-765fed38bc5c"
      },
      "cart": "https://example.com/",
      "campaign": "https://example.com/",
      "excluded": false,
      "cartItemExtras": [
        {
          "@context": "string",
          "@id": "string",
          "@type": "string",
          "extra": {
            "@context": "string",
            "@id": "string",
            "@type": "string",
            "description": "string",
            "conditions": {},
            "name": "string"
          },
          "price": {}
        }
      ]
    }
  ]
}