restful mapping 支持映射 http method

**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)**

restful 映射时添加 METHOD 支持, 类似:
```
POST /item => create_item,
DELETE /item => delete_item,
PUT /item => put_item,
GET /item => get_item
```

**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)**

Start by locating brpc's RESTful mapping entry point and any existing route-mapping tests. Implement support for distinguishing POST, DELETE, PUT, and GET routes for the same path, then verify that each method dispatches to its corresponding handler.

Written by the indexing model from the issue text.