Support uploading custom keyboard layouts

[ah-]

Implement the necessary protocol bits. Especially probably handling larger message sizes.

Then somehow convert from the obins layout format (I think it's 2 bytes per key, one with the scancode for the base layer, one for fn layer) to the internal Action based format.

Think about how we want to handle Actions that don't have an obins scan code, like theme switching and layer activation.

And then check how we can persistently store keymaps, maybe in nvram/eeprom or in a special flash region?