Refactor module options and support rootless k3s & nix-snapshotter

[elpdt852]

Fixes #83

Highlights

• Provides NixOS & Home Manager modules for rootless Kubernetes (k3s) + nix-snapshotter
• Bumps version to 0.2.0
• Separate nix run .#vm and nix run .#vm-rootless
• Add integration tests for k3s, k3s-external, k3s-rootless
• Separate preload-container service into independent modules
• Pin k3s to v1.27.9+k3s1 with patches to enable embedded nix-snapshotter
  • k3s PR to support nix-snapshotter: https://github.com/k3s-io/k3s/pull/9319
• Add k8sResources flake perSystem output and plumb into specialArgs

New options

• Services preload-containerd & preload-containerd.rootless:

  config.services.preload-containerd = {
  enable = true;
  targets = [{
    archives = [ pkgs.nix-snapshotter.buildImage { /* ... */ } ];
    namespace = "k8s.io";
    address = "/run/k3s/containerd/containerd.sock";
  }];
  };

• New options for k3s & new service k3s.rootless:

  config.services.k3s = {
  enable = true;
  # Sets the snapshotter for embedded containerd.
  snapshotter = "nix";
  # Sets KUBECONFIG env var to point to k3s.
  setKubeConfig = true;
  # Sets CONTAINERD_* env vars to point to k3s embedded containerd.
  setEmbeddedContainerd = true;
  }

• New options for containerd & containerd.rootless:

  config.virtualisation.containerd = {
  enable = true;
  # Enable integration with nix-snapshotter.
  nixSnapshotterIntegration = true;
  # Set the CONTAINERD_* env vars, but also set automatically by
  # `nixSnapshotterIntegration` or by `services.k3s.setEmbeddedContainerd`.
  setAddress = "/run/containerd/containerd.sock";
  setNamespace = "default";
  setSnapshotter = "nix";
  }

• New option only for NixOS module containerd:

  config.virtualisation.containerd = {
  enable = true;
  # Enable integration with k3s. This is mutually exclusive with setting
  # `services.k3s.snapshotter` and `services.k3s.setEmbeddedContainerd`.
  k3sIntegration = true;
  };

Migration guide v0.1.x -> v0.2.0

• Removed options.services.nix-snapshotter.setContainerdSnapshotter

  # v0.1.x
  services.nix-snapshotter = {
  enable = true;
  setContainerdSnapshotter = true;
  };
  
  # v0.2.0 (same for rootless)
  virtualisation.containerd = {
  enable = true;
  nixSnapshotterIntegration = true;
  };
  services.nix-snapshotter = {
  enable = true;
  };

• Removed options.services.nix-snapshotter.preloadContainerdImages

  # v0.1.x
  services.nix-snapshotter = {
  enable = true;
  preloadContainerdImages = [ pkgs.nix-snapshotter.buildImage { /* ... */ } ];
  };
  
  # v0.2.0 (same for rootless)
  virtualisation.containerd = {
  enable = true;
  nixSnapshotterIntegration = true;
  }
  services.nix-snapshotter = {
  enable = true;
  };
  services.preload-containerd = {
  targets = [{
    archives = [ pkgs.nix-snapshotter.buildImage { /* ... */ } ];
  }];
  };