[FEATURE] allow entry point overriding

Description of the problem being solved

There is no way for developers to override the entry point file name/path (default: app.py)

Alternative solutions

Open to ideas

Simple solution: Set an environment variable to override the default entry point value

Requirements

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首先定位默认入口点 app.py 的定义位置，以及 Slack CLI 如何传递或使用该值。检查提议的环境变量方案和相关的入口点处理；当开发者可以在不改变默认行为的情况下选择不同的文件名或路径时，即视为完成。