Add performace analytics group level leaderboard api

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Add performance analytics group level leaderboard api

1. 榜单接口 groups/*group_id/-/analytics/performance_analytics/leaderboard.json

   参数格式

   • project_ids 项目ID数组
   • start_date 查询开始日期(查询时转化为UTC日期的开始时间00:00:00)
   • end_date 查询结束日期(查询时转化为UTC日期的结束时间23:59:59)
   • leaderboard_type 榜单类型 commits_pushed issues_closed merge_requests_merged

   返回数据

   [
     { 
        "user": {
           "fullname": "Linjie Zhang",
           "username": "zhanglinjie",
           "user_web_url": "/zhanglinjie",
           "avatar": "/uploads/-/system/user/avatar/290/avatar.png?width=30"
        },
        "rank": 1,
        "value": 100
     } 
   ]