Merge the audio_frontend module into the microlite module

[mocleiri]

Originally I had the audio_frontend as a separate module to allow for not including it but there are other ways to do so. For circuit python compatibility I think I need to have everything in the microlite module.

Lets restructure things so that audio_frontend is a class within the microlite module. I think ulab has an example we can use for conditionally including the audio_frontend class.

[mocleiri]

I've made this work and merged this branch down to main. This branch had the stm32 and rp2 ports on it and is now back into the mainline.