feat(flow-rs): use ffmpeg dylib

[tpoisonooo]

因 pyo3 问题， 对 macos 支持还在修复。

• 修改 ffmpeg 为 .so 或 .dylib 调用，满足 LGPL 要求
• 建图支持 dict ，即 Python 支持 map_reduce 类用法
• 调整 Python 相关目录结构，区分应用范例和 megflow 本身用法的示例
• 新增编译选项 open-camera，可以在 VideoServer 输入 /dev/video0 直接打开摄像头，此时不需要 RTSP
• github 文档调整风格，支持到 docusaurus 风格转换。即一份文档可以两种部署

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