Modular services tracking

[roberth]

Tracking issue for Modular Services.

• Docs as of nixos-unstable
• Iniital PR

Known issues:

• docs generation for importable modules (assigned: @roberth @mightyiam)
  • nixos/documentation/modular-services: init #430876
    • improve, nixos/ghostunnel: Documentation rendering bug #432550
  • something more structured, at least for the manual
  • make search.nixos.org backend responsible for combining the NixOS and service option sets as it sees fit
• reloadTriggers
• environment variables

Next steps. Can be reordered as needed.

• 1. Make initial implementation Modular services #372170 available for contributors
• 2. Equip modular services with functionality to support more kinds of services (i.e. so that more projects run well on it)
• 3. Integration into e.g. nix-darwin, arion, devenv?
• 4. Use modular service to implement traditional service and distill instructions
• 5. Evaluate state of affairs
• 6. nix.dev docs
• 7. Discourse post

Other TODOs:

• systemd user unit support
• try combine with Contracts RFC 189 #432529

[KiaraGrouwstra]

i think one relevant todo to add here would be providing a transition path so services could be ported in a way the 'legacy module' interface would become a wrapper over the portable service module, e.g. options.services.ghostunnel = pkgs.ghostunnel.services.default;.

edit: a PR on this had since been opened at #475372