Yubikey support

Having just bought a 2-pack of the Yubikey 5 NFC, I've spent some time setting up Juno for Yubikey authentication (full disk encryption, system-wide login, etc) and documenting the process in https://github.com/carniz/ubuntu-yubikey-setup

I'd like to see support for Yubikey in the installer, and I visualize a step where you get to enroll your Yubikey(s) for full disk encryption and local login (lightdm + TTYs). This should probably require that the keys are already configured for OTP challenge-response, but it could probably be done also by the installer.

End result: the most secure and privacy-respecting OS there is!