Add a flag to enable/disable reload mock config automatically

The [Mock Server](https://github.com/LinuxSuRen/api-testing/blob/ffac34fd9dc373bb5a2c7b56f54fae2efd52ebf4/cmd/mock.go) can only read the config at the startup stage. Users need to restart it if they modified the config file content.

## Solution
Support to watch the changes of the mock config file. And provide a flag to enable or disable this feature. The usage might be:

```shell
atest mock --files demo-server.yaml --auto-reload [true|false]
```

And I prefer setting the flag value `--auto-reload` to be `false`.
## References
* https://medium.com/@skdomino/watch-this-file-watching-in-go-5b5a247cf71f