yubikey-agent: init at 0.1.3

This adds yubikey-agent as a package and a nixos module.

On macOS, we use `wrapProgram` to set pinentry_mac as default in PATH;
on Linux we rely on the user to set their preferred pinentry in PATH.
In particular, we use a systemd override to prefix PATH to select a
chosen pinentry program if specified.

On Linux, we need libnotify to provide the notify-send utility for
desktop notifications (such as "Waiting for Yubikey touch...").

This might work on other flavors of unix, but I haven't tested.

We reuse the programs.gnupg.agent.pinentryFlavor option for
yubikey-agent, but in doing so I hit a problem: pinentryFlavour's
default value is specified in a mkDefault, but only conditionally.  We
ought to be able to pick up the pinentryFlavour whether or not gpg-agent
is running.  As a result, this commit moves the default value to the
definition of programs.gnupg.agent.enable.

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