keyboard-service: Come up with strategy for handling special keys/fn keys

Handling special keys and function keys in a broad, general sense is tricky because typically special keys correspond to sending special reports such as consumer page reports (whereas most standard keys are all part of the basic input report) or firmware-specific behavior.

Would need to come up with a way to allow user to define special keys, what action should be taken if pressed (maybe via callback/closure?) and also allow user to inject non-input report formats into the report descriptor and generate the appropriate report when the key is pressed.

There doesn't appear to be a great solution to this, so may just need to have users implement the HidKeyboard trait themselves for all the custom/specific behavior they need. But I will continue to think about this some more.