允许SDK用户指定自定义的http.Transport

[ricklxm]

能否在core/config/http_config.go中的HttpConfig结构体中增加字段以便用户使用自定义的http.Transport?

type HttpConfig struct {
	DialContext           DialContext
	Timeout               time.Duration
	Retries               int
	HttpProxy             *Proxy
	IgnoreSSLVerification bool
	HttpHandler           *httphandler.HttpHandler
	**HttpTransport         *http.Transport //新增字段** 
}