code: func Proxy(proxy string) Option {
  	return func(gsa *GServiceAccount) error {
  		u, err := url.Parse(proxy)
  		if err != nil {
  			return err
  		}
  		return Transport(&http.Transport{
  			Proxy: http.ProxyURL(u),
  		})(gsa)
  	}
  }
docs: // Proxy is a GServiceAccount option to set a HTTP proxy used for by the token
  // source.