服务端API，自动提供req.user 变量供调用
Created by: hotlong
调用方法参考： https://steedos.github.io/docs/en/plugin_developer#定义服务端api
如果当前用户未登录, req.user = null
Created by: hotlong
调用方法参考： https://steedos.github.io/docs/en/plugin_developer#定义服务端api
如果当前用户未登录, req.user = null