grub: Add `boot.loader.grub.extraGrubInstallArgs` option.

Useful for when you need to build grub modules into your grub kernel
to get a working boot, as shown in the added example.

To store this new value, we switch to more structural JSON approach.

Using one line per value to store in `/boot/grub/state` gets really messy when
the values are arrays, or even worse, can contain newlines (escaping would be
needed). Further, removing a value from the file would get extra messy
(empty lines we'd have to keep for backwards compatibility).

Thus, from now on we use JSON to store all values we'll need in the future.