List available selling plan groups

{
  "success": true,
  "message": "Selling plan groups fetched successfully",
  "data": [
    {
      "id": 58666,
      "name": "Subscribe for monthly delivery",
      "position": 1,
      "merchantCode": "Subscribe for monthly delivery",
      "options": [
        "Frequency"
      ],
      "summary": "1 delivery frequency, 30% discount",
      "createdAt": "2026-06-09T07:28:04.000Z",
      "updatedAt": "2026-06-15T09:44:15.000Z",
      "sellingPlans": [
        {
          "id": 124835,
          "shopifyId": 9603711283,
          "name": "Deliver every month",
          "options": [
            "Deliver every month"
          ],
          "pricingPolicies": [
            {
              "discountType": "PERCENTAGE",
              "discountValue": 30
            }
          ],
          "billingPolicy": {
            "interval": "MONTH",
            "intervalCount": 1,
            "anchorDay": 5,
            "anchorType": "MONTHDAY"
          },
          "deliveryPolicy": {
            "interval": "MONTH",
            "intervalCount": 1
          },
          "isDefault": true
        }
      ]
    }
  ],
  "code": "SUCCESS"
}