feat: add flake-parts installer image options

[different-name]

Creating an offline installer configuration is already demonstrated in the disko docs, however it's pretty dense and I think a module would be a great replacement for copying that large chunk of code from the docs

This PR adds the disko-install option set to the flake-parts module, which generates installer images with dependencies & install script included

Minimal config:

disko-install.hosts = {
  my-host.disks.main = /dev/disk/by-id/some-disk-id;
};

Exhaustive config:

disko-install.hosts = {
  my-host = {
    nixosConfiguration = "my-host";
    disks.main = /dev/disk/by-id/some-disk-id;
    image = {
      formats = [ "iso-installer" "hyperv" ];
      modules = [ /*<nixos module>*/ ];
    };
    script = {
      name = "my-cool-installer";
      extraArgs = [ "--dry-run" ];
    };
  };
};

TODO

This works to generate installer images (I have tested with iso-installer) with a install-nixos-unattended script, however it still needs work:

• Stricter typing for formats option, as it currently is just listOf str
• Documentation
• Tests?

[different-name]

Marking this as ready for review, since the install script is implemented & I'm happy with the way it's configured

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/offline-automated-installation-images-for-disko/75953/1

[mightyiam]

How about some tests?

[different-name]

Tests is on the todo above, I don't have much experience with CI, and wanted to gauge if this is something desirable before continuing with docs & tests