Is it possible to stream output?

Right now `execnb` is waiting for the final response from code execution. Is it possible to steam execution results without waiting for final response?

I'm using `execnb` for executing notebooks in the [Mercury](https://github.com/mljar/mercury) framework, and got such question from user https://github.com/mljar/mercury/issues/301

未指定文件或测试。首先跟踪 execnb 的 notebook 执行入口点以及它如何等待最终响应，然后确定可以在哪里公开增量结果。当执行结果在完成之前交付，并且针对 Mercury 用例演示了该行为时，即视为完成。

由索引模型根据 Issue 内容生成。