Support passing through HID events even if they aren't part of the spec

[spoelstraethan]

I have a Brydge Type-C keyboard that was designed for Chromebooks but works great for Android and Linux and even Windows, but some of the keys it sends weren't recognized by RPI-KVM even though they may have been understood by the target OS. It would be nice to pass through unknown events especially if they aren't ones that would be expected for hotkeys because they aren't common to all keyboards, and it would also be nice if it were possible to allow a user to add custom key codes to the hotkey options manually especially if they had something like KEY_CONTROLPANEL that sends a HID code that is valid and seen but isn't recognized by rpi-kvm.

An excellent example of this is the tool Nikau by NickBP on sr.ht that totally redirects any input devices not in the filter list to whatever specific host is targeted, and the hotkey to switch is actually a chord of two keys pressed in sequence so it is easier to avoid conflicts with any shortcuts mapped on the target/client systems or the host system where the keyboard and mouse are attached.