[BREAKING] feat: support loadConfig asynchronous

[gxcsoccer]

Checklist

• npm test passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

Description of change

oss 的 ak/sk 可能配置的是加密的，启动的时候可能需要先调用 api 进行解密

exports.oss = {
  default: {
    async loadConfig(config, app) {
      const accessKeyId = await app.kmiclient.decrypt(config.accessKeyId);
      const accessKeySecret = await app.kmiclient.decrypt(config.accessKeySecret);
      return Object.assign({}, config, {
         accessKeyId,
         accessKeySecret,
      });
    },
  },
  client: {
     accessKeyId: '**********',
     accessKeySecret: '***********',
     endpoint: 'http://oss-cn-hangzhou.aliyuncs.com',
     bucket: 'xxx',
  },
};

[codecov]

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Additional details and impacted files

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##             master       #11   +/-   ##
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  Coverage          ?   100.00%           
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  Files             ?         6           
  Lines             ?        33           
  Branches          ?         0           
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  Hits              ?        33           
  Misses            ?         0           
  Partials          ?         0           

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[troyeagle]

这样直接赋值的话 client 的配置会不会覆盖 default 啊（盲猜
测试用例里好像也没体现

[gxcsoccer]

https://github.com/eggjs/egg-oss/pull/11/files#diff-fea80874fae611c4cc788e04a55d51a9R8

这里吧

[troyeagle]

这样的话 default 中的异步函数由于延迟执行，会覆盖 client 里面已经设置好的重复字段；对于 client 和 default 都需要异步函数的情形还支持不了。
https://gist.github.com/troyeagle/b10774eff060cc6c6d1ed51ba8d842ec 可能是一种解决方法。总之用异步函数设定的值会打破原先 default 和 client 的加载顺序。求教在这种情形下应该怎么考虑设计