Allow `ChatAuto()` listen to one envvar

Similar to how SQLAlchemy does it, something like 

```shell
export CHATLAS_MODEL=anthropic://claude-3-haiku-20240229?max_retries=3
export ANTHROPIC_API_KEY=your_api_key
```

And also support [entry points](https://packaging.python.org/en/latest/guides/creating-and-discovering-plugins/#plugin-entry-points) for extension packages to plugin to the system