Add ability to configure nix settings via module

[EliSauder]

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

Currently, system manager cannot configure nix settings like experimental features or garbage collection. I understand that nix determinate has some of these features, however, I prefer manually having it configured to exactly what my need is rather than rely on someone else's configuration.

Describe the solution you'd like

The ability to enable experimental features and garbage collection using a module. e.g.

nix = {
  settings.experimental-features = [
    "nix-command"
    "flakes"
  ];
  gc.automatic = true;
};

Describe alternatives you've considered

Manually configure nix.conf file using etc module.

Additional context