[Research] Figure out how to migrate the API to a snekomatic-style shared-maintenance lib

[webknjaz]

Nathaniel Smith has designed quite a beautiful API for representing GH App/Installation entities back in August. I'd like octomachinery to reuse them partially and maybe offload some parts to a third-party lib based on that.

The idea is to have a separate lib that would be octomachinery's dependency so that:

• we'd have a shared code maintenance
• the API's UX/DX would be great
• it'd partially help avoid asyncio lock-in allowing it do be used in a broader variety of envs

For this, octomachinery needs some refactoring that I've started already. Also, we need to figure out what would belong to that library and what would have to stay inside of octomachinery.

Ref: https://github.com/python-trio/snekomatic/blob/13e233c/snekomatic/gh.py

[webknjaz]

@ekohl ^