`system.autoUpgrade.*` Functionality

[commiterate]

Is your feature request related to a problem? Please describe.

Users currently need to add their own auto-update infrastructure to periodically update non-NixOS Linux machines managed with system-manager.

It would be convenient if system-manager could auto-create systemd units which periodically do a system-manager switch.

Describe the solution you'd like

NixOS has system.autoUpgrade.* options which create a systemd timer that periodically runs nixos-rebuild switch.

The idea is to support most of the same system.autoUpgrade.* options (maybe not the reboot ones since system-manager doesn't manage the kernel or initrd yet) but generate a systemd timer which runs system-manager switch instead of nixos-rebuild switch.

Describe alternatives you've considered

Writing the unit files manually outside of system-manager essentially equivalent to the below NixOS module but without the special NixOS systemd unit configurations controlling unit removal/update behavior.

{
  lib,
  pkgs,
  ...
}:
{
  config = {
    systemd = {
      services = {
        system-manager-upgrade = {
          description = "system-manager Upgrade";
          after = [
            "network-online.target"
          ];
          wants = [
            "network-online.target"
          ];
          unitConfig = {
            X-StopOnRemoval = false;
          };
          serviceConfig = {
            X-RestartIfChanged = false;
            Type = "oneshot";
            ExecStart = "${lib.meta.getExe system-manager} switch --flake {flake URI} --refresh";
          };
        };
      };

      timers = {
        system-manager-upgrade = {
          timerConfig = {
            FixedRandomDelay = false;
            OnCalendar = "daily";
            Persistent = true;
            RandomizedDelaySec = 0;
          };
        };
      };
    };
  };
}

I think these units currently can't be managed by system-manager since it doesn't handle the special lifecycle attributes.

Additional context

Likely needs #129 to be addressed first to add a --refresh flag to system-manager commands (forcefully re-resolve Git references like refs/heads/main).