func (c *RPCCtx) SetResultCbExt(cb CallbackExt) {
	c.cbExt = cb
}
SetResultCbExt extension rpcctx  callback
package jsonclient
import (
	"encoding/json"
	"fmt"
	"os"
)
type RPCCtx struct {
	Addr   string
	Method string
	Params interface{}
	Res    interface{}
	cb     Callback
	cbExt  CallbackExt
}
type Callback func(res interface{}) (interface{}, error)
type CallbackExt func(res ...interface{}) (interface{}, error)
func NewRPCCtx(laddr, method string, params, res interface{}) *RPCCtx {
	return &RPCCtx{
		Addr:   laddr,
		Method: method,
		Params: params,
		Res:    res,
	}
}
func (c *RPCCtx) SetResultCb(cb Callback) {
	c.cb = cb
}