curl --request GET \
  --url http://localhost/api/v3/profiles/{profile}/lists/{list}/custom-fields

{
  "data": [
    {
      "name": "<string>",
      "handle": "<string>",
      "namespace": "<string>",
      "value": "<string>",
      "type": "<string>",
      "custom_fieldable_type": "<string>",
      "custom_fieldable": {
        "id": "<string>",
        "external_id": "<string>",
        "profile_id": "<string>",
        "meta": "<string>",
        "total": "<string>",
        "total_original": "<string>",
        "rounding": "<string>",
        "margin": "<string>",
        "is_void": "<string>",
        "transacted_at": "<string>",
        "timezone": "<string>",
        "payments": "<string>",
        "receipt_is_email": "<string>",
        "receipt_ref": "<string>",
        "linked_receipt_ref": "<string>",
        "claimed_at": "<string>",
        "receipt_email": "<string>",
        "currency_id": "<string>",
        "currency_rate": "<string>",
        "currency": "<string>",
        "type": "<string>",
        "status": "<string>",
        "order_number": "<string>",
        "order_id": "<string>",
        "external_order_id": "<string>",
        "created_at": "<string>",
        "updated_at": "<string>"
      },
      "created_at": "<string>",
      "updated_at": "<string>",
      "version": 123,
      "is_index": 123
    }
  ]
}