boot_command syntax for sending custom key codes

[mcandre]

FOR FEATURES:

The packer boot_command syntax is far from complete for describing the kinds of keys that can be sent to guest VM's.

I'd like to see PR's to arrive for <leftCmdOn>/<leftCmdOff> (Command key in macOS), <leftWinOn>/<leftWinOff> (Windows key in Windows), <leftSupOn>/<leftSupOff> (generic Super key in macOS/Windows/Linux). In the meantime, however, it would suffice if packer simply offered some kind of syntax for entering raw keycodes within boot_command, perhaps something like <raw n>, that packer interprets as sending the decimal number n keycode to the hypervisor. This way, packer handles keyboard input much more flexibly, so that users don't have to wait around for PR's to handle more obscure key codes.

[rickard-von-essen]

Windows/Command/Super keys are all called L_GUI/R_GUI in keyboard nomenclature.

[SwampDragons]

Whoever takes this on should probably tackle this at the same time as #5635

[mwhooker]

this should easily be doable once #6129 is in. I would be happy to assist with this