feat: Add micropython bindings

[Willaaaaaaa]

Add micropython interfaces in subdir micropython. Use micropython as a submodule.

ORIGIN REPO: https://github.com/Willaaaaaaa/ncnn_mp (which uses ncnn as a dependence)

[github-actions]

The binary size change of libncnn.so (bytes)

architecture	base size	pr size	difference
x86_64	15046688	15643128	+596440 ⚠️
armhf	6081956	6648220	+566264 ⚠️
aarch64	9387400	9986896	+599496 ⚠️

[Willaaaaaaa]

I have modified the existing code in my own repo.
I'm planing to update this PR when I have fully implemented this task. So plz give my any advice based on my repo instead of this PR(or I can update this PR now if you want). Thanks a lot!

see my current modification plan and mid-term report: Willaaaaaaa/ncnn_mp#1

[nihui]

感谢你的工作，请将你在实现中的笔记和心得，遇到的困难和解决方法等，记录成文章，发表在discussion分区，这将作为知识总结 https://github.com/Tencent/ncnn/discussions

Thank you for your work. Please record your notes and experience in the implementation, difficulties encountered and solutions, etc. into an article and publish it in the discussion section. This will serve as a knowledge summary. https://github.com/Tencent/ncnn/discussions

[Willaaaaaaa]

The code I update is originally from my repo: ncnn_mp

My Work

1. Completed the api design and implementation.
2. Added a .pyi stub file to provide hints, definitions and docstrings.
3. Created examples for new-comers: mnist.py, squeezenet.py and test_api.py.
4. Provided build instructions and examples in README.

For more infomation

• Relative Discussion: ncnn_mp — ncnn C API bindings for MicroPython #6292
• See issues and prs in my repo.