Add examples to volo / 为 Volo 增加一些示例

## Feature Request

### Proposal

Firstly, we need to create a new directory called `examples`, and write an idl file.

Secondly, we should generate an example using volo-cli, and adjust the `Cargo.toml` to use path instead of from `crate.io`.

Thirdly, we need to add an example to write a layer such as log layer, and add it to the server or client.