nixos/top-level: add specialisation.<name>.configuration type

[Scriptkiddi]

Motivation for this change

#76184

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[cole-h]

Diff LGTM, thanks!

[ajs124]

I'm not sure why or how, but with this, the nginx test in nixos/tests fails to evaluate with:

error: The option `services.nginx.virtualHosts.localhost.root' has conflicting definition values:
- In `<unknown-file>': <derivation /nix/store/pbfyh5b86m14f3vkqz7gb8anfiyz2bmc-testdir.drv>
- In `<unknown-file>': <derivation /nix/store/jz45f8nkvqb82iim1s192349c28lbm4b-testdir2.drv>
(use '--show-trace' to show detailed location information)

cc @infinisil as you're the specialisation. guy AFAIR

[ajs124]

Yeah, the acme and specialisation test are also broken with this.

[cole-h]

Sorry, I guess I didn't run the related test this time...

I don't actually know how to type this. types.anything works, but I don't know if we want to be more specific.

Ping our resident type-master @infinisil.

[roberth]

#143207 can make this a proper submodule type.