code: function formatProxyConfig(config) {
    const res = Object.assign({}, config);
  
    const serviceConfig = {};
    const SERVICE_CONFIG = [ 'errorAsNull', 'responseTimeout' ];
  
    for (const key in config) {
      if (SERVICE_CONFIG.indexOf(key) > -1) {
        serviceConfig[key] = config[key];
      }
    }
    res.services = !res.services ? [] : res.services.map(service => {
      const target = Object.assign({}, serviceConfig, service);
      assert(target.appName, '[egg-rpc-generator] service.appName is required in proxy.js');
      return normalizeService(target);
    });
    return res;
  }
docs: format proxy config
  1. 将 errorAsNull, responseTimeout 放到 service 中
  2. api.key 对象化， dep 数组化
  @param  {Object} config config
  @return {Object}        format result