Add performance analytics group level summary api

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Splited from MR

Add performance analytics group level summary api

1. 总面板接口 groups/*group_id/-/analytics/performance_analytics/summary.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)

   返回数据

   {
     "issues_closed": 0, 
     "commits_pushed": 0,
     "merge_requests_merged": 0,
     "commits_pushed_per_capita": 0
   }