server支持OnNewConnection(...)， OnClosedConnection(...)回调

**Is your feature request related to a problem? (你需要的功能是否与某个问题有关?)**
我们在用户层面实现了基于Connection ： Session = 1: 1的功能，需要在connection建立和断开时对session进行保存和删除。

**Describe the solution you'd like (描述你期望的解决方法)**
希望ServerOption提供OnNewConnection, OnClosedConnection的回调, 以便在用户代码中管理连接信息。

Start by locating ServerOption and the server connection lifecycle in the brpc source. Trace where connections are established and closed, then review existing tests or callback conventions before defining the callback behavior. Done means both callbacks are exposed through ServerOption, invoked at the appropriate lifecycle points, and covered by tests.

Written by the indexing model from the issue text.