baidu-std协议支持传输任意格式的数据

**现状说明**
baidu-std协议目前支持[传输任意类型的request](https://github.com/apache/incubator-brpc/blob/master/src/brpc/protocol.cpp#L132)，但是不支持解析任意类型的response，目前内部版本已经实现了baidu-std协议下支持传输任意类型的数据格式

**需求描述**
用户可以通过baidu-std协议传输任意格式的数据，比如：string，json等等，用户自己对数据进行打包和解包，rpc框架不需要关心

**实现方案**
- 新增SerializedResponse类（参考SerializedRequest）
- 在baidu-std协议ProcessRpcResponse函数中判断 cntl->response()->GetDescriptor() == SerializedResponse::descriptor()

Start in src/brpc/protocol.cpp around the existing arbitrary-request handling, then compare the proposed SerializedResponse class with SerializedRequest. Trace baidu-std's ProcessRpcResponse and the response descriptor check. Done means baidu-std can carry formats such as strings or JSON while callers handle packing and unpacking.

Written by the indexing model from the issue text.