Idea for CPU bound task hosted by brpc

**Is your feature request related to a problem? (你需要的功能是否与某个问题有关?)**

**Describe the solution you'd like (描述你期望的解决方法)**
希望能方便利用多线程处理CPU计算任务。

比如，在brpc中使用C++ executors会和brpc的内部的线程模型冲突吗？ 或者brpc提供了自己的多线程编程接口吗？

**Describe alternatives you've considered (描述你想到的折衷方案)**

**Additional context/screenshots (更多上下文/截图)**

No source file, test, or entry point is named. Start by reading brpc's internal thread model and any existing CPU-task or executor interfaces, then determine whether C++ executors conflict with them or whether brpc already provides a suitable API. Done would require a decided, documented approach for handling CPU-bound work.

Written by the indexing model from the issue text.