[Performance Analytics] Api for 'project' leaderboard

Api for 'project' leaderboard

Leaderborad api path *namespace_id/*project_id/-/analytics/performance_analytics/leaderboard.json

params:

• branch_name
• leaderboard_type 榜单类型 commits_pushed issues_closed merge_requests_merged
• start_date
• end_date

return data:

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