Draft: bpf: Add support for the Huion KeyDial K20

Closes #35

Replaces !144 - currently this only replaces the vendor mode, not yet the firmware node descriptors. That's intentional because I'm planning to use this as reference MR for some documentation, the firmware support comes as a follow-up commit.

The most notable thing here is that the dial and dial button (the modeswitch button) do not work in firmware mode. They send events on the hidraw node but those are unreliable: the button does not release and even if it does it may do so only after several dial clicks. Likewise the dial sends garbage (like 0 values) and often does not change direction until several clicks late. Neither event is forwarded to userspace, the whole hidraw node is ignored by the kernel.

cc @Deevad