1841 获取树状分类数据接口 App.Table.FreeTree

假设有表单网站菜单：website_menu

表单数据有：
+----+--------------+-----------+
| id | title        | parent_id |
+----+--------------+-----------+
|  1 | 首页         |         0 |
|  2 | 接口         |         0 |
|  3 | 通用接口     |         2 |
|  4 | 插件接口     |         2 |
|  5 | 文档         |         0 |
|  6 | 接口文档     |         5 |
|  7 | 平台文档     |         5 |
|  8 | 联系我们     |         0 |
+----+--------------+-----------+

请求：
{你的接口域名}/?s=Table.FreeTree&model_name=website_menu&parent_id=0&select=title&app_key={你的app_key}&sign={动态签名}

返回：
{
    "ret": 200,
    "data": {
        "err_code": 0,
        "err_msg": "",
        "tree": [
            {
                "title": "首页",
                "id": 1,
                "parent_id": 0,
                "children_tree": []
            },
            {
                "title": "接口",
                "id": 2,
                "parent_id": 0,
                "children_tree": [
                    {
                        "title": "通用接口",
                        "id": 3,
                        "parent_id": 2
                    },
                    {
                        "title": "插件接口",
                        "id": 4,
                        "parent_id": 2
                    }
                ]
            },
            {
                "title": "文档",
                "id": 5,
                "parent_id": 0,
                "children_tree": [
                    {
                        "title": "接口文档",
                        "id": 6,
                        "parent_id": 5
                    },
                    {
                        "title": "平台文档",
                        "id": 7,
                        "parent_id": 5
                    }
                ]
            },
            {
                "title": "联系我们",
                "id": 8,
                "parent_id": 0,
                "children_tree": []
            }
        ]
    },
    "msg": "小白开放接口：App.Table.FreeTree"
}