How to create custom unmarshal for 2 types map[string]interface{} and []interface{}

Here's my sample go playground https://go.dev/play/p/MosQs62YPvI

My curl API return 2 kind of return, can any of the ff:

{
  "code": 200,
  "message": "Success",
  "data": {
    "list": {
      "1": {
        "user": "user A",
        "status": "normal"
      },
      "2": {
        "user": "user A",
        "status": "normal"
      }
    },
    "page": 1,
    "total_pages": 2000
  }
}

or

 {
  "code": 200,
  "message": "Success",
  "data": {
    "list": [
      {
        "user": "user A",
        "status": "normal"
      },
      {
        "user": "user B",
        "status": "normal"
      }
    ],
    "page": 1,
    "total_pages": 5000
  }
}

How to unmarshal it properly?

Here's my struct

    type User struct {
	Code    int    `json:"code"`
	Message string `json:"message"`
	Data    struct {
		List []struct {
			User   string `json:"user"`
			Status string `json:"status"`
		} `json:"list"`
		Page       int `json:"page"`
		TotalPages int `json:"total_pages"`
	} `json:"data"`
}

Here's how I unmarshal it

err = json.Unmarshal([]byte(io_response), &returnData)
	if err != nil {
		log.Println(err)
	}

I have tried creating my own unmarshal but I have issues converting it to map[string]interface{}

Can you please help me? Or is there any better way?

type UserItem struct {
	User   string `json:"user"`
	Status string `json:"status"`
}

type UserList []UserItem

func (ul *UserList) UnmarshalJSON(data []byte) error {
	switch {
	case len(data) == 0 || string(data) == `null`:
		return nil
	case data[0] == '[': // assume it's a JSON array
		return json.Unmarshal(data, (*[]UserItem)(ul))
	case data[0] == '{': // assume it's a JSON object
		obj := make(map[string]UserItem)
		if err := json.Unmarshal(data, &obj); err != nil {
			return err
		}
		for _, v := range obj {
			*ul = append(*ul, v)
		}
		return nil
	default:
		return fmt.Errorf("unsupported json type")
	}
	return nil
}

https://go.dev/play/p/Y5PAjrmPhy2