installer: add support for flatpakref.

[gmodena]

This is a WIP.

Issue #78

Adds support for installing application from a flatpakref uri.

Changes

When installing from a flatpakref, and packages will tracked in the gcroot state files:

1. a package will be stored, with appId and origin set to the flatpakref's Name and SuggestRemoteName respectively.
2. a remote with be stored labelled with SuggestRemoteName.

This should enable compatibility with state management APIs.

Testing

Workflow:

1. declare a flatpakref package.
2. on activation, the package and its remote are installed.
3. remove the flatpakref declaration.
4. on activation, the package and it's remote are removed.

• Tested on a testing-base nixos VM , with nix-flatpak loaded as a home-manager module.
• Tested on a testing-base nixos VM , with nix-flatpak loaded as a nixos module.

services.flatpak.remotes = lib.mkOptionDefault [{
   name = "flathub-beta";
   location = "https://flathub.org/beta-repo/flathub-beta.flatpakrepo";
};

services.flatpak.packages = [
    { 
      flatpakref="https://sober.vinegarhq.org/sober.flatpakref";
      sha256="1pj8y1xhiwgbnhrr3yr3ybpfis9slrl73i0b1lc9q89vhip6ym2l";
    }
  ];

Example state config.

cat /nix/var/nix/gcroots/flatpak-state.json:

{
    "overrides": {},
    "packages": [
       "org.vinegarhq.Sober"
    ],
    "remotes": [
        "flathub",
        "flathub-beta",
        "sober"
    ]
}