Support uploading custom LED themes in Keyboard stm32

[ah-]

Once we can pair to the App it'd be nice to be able to support uploading custom LED themes.
I think all we need to do in the key stm32 is forward the right messages from bluetooth receive to the LED chip.

This might also require a bit of refactoring of the serial support and increasing buffer sizes since those themes are way larger than the data we handle so far.

[hdhoang]

In obins' v0.13, there is both a KEY dfu and an LED dfu, with obinslab support. I have yet to flash them. Hopefully the key<->led protocol is similar to the current one, and we just need to add obinslab<->key protocol as outlined in this issue.