keyboard-service: Add support for key scanning and HID communications

[jeffglaum]

Port and as appropriate, refactor the existing implementation. Features:

• software-drive key matrix scanning
• support for hardware matrix scanners (example: Microchip MEC17xx)
• key debounce/filtering with tuning ability for different key types
• key ghosting
• support for caps lock and keyboard function LEDs
• efficient interface to HID service for host-side communications
• separate configuration (ex: key layout, debouncing parameters, etc.) from logic for easy customization

[kurtjd]

Most of these items are now resolved via #510 (with the exception of key LEDs).
#499 will be used to track more specific work that needs to be done, and that will further be broken out into sub-issues as I come up with a better plan to tackle them.