Mock Backend for Testing/etc

[AstraLuma]

Idea: Have a mock backend for when you want to use Bleak in synthetic contexts.

Basically, a backend that instead of connecting to real hardware, allows tools to load fake data. This would allow end-to-end automated testing of libraries using Bleak without having to rely on fragile monkey patching.

[AstraLuma]

I've started on a version of this in my own project https://github.com/AstraLuma/nat20/tree/trunk/pytest_bleak