feat: add authentik service

[gabyx]

I tried to configure an authentik service with process-compose based on the packages in https://github.com/nix-community/authentik-nix. I was quite successful I think.

• Documentation
• Tests health point + blueprint export)
• Export the blueprints (db dumb etc) with blueprints.export.enable = true and triggering the process.
• The modules does not pull in the authentic-nix flake but requests it on components option.
• authentik service needs a sidecar services postgres.
  These are easily available configured already under services.postgres. See the documentation and the tests.
  Note: This implementation actually posed the question how an authentik module can configure another service as a sidecar. Currently how the modules are loaded we have no access to the overall toplevel options. @shivaraj-bh (maybe there is a better option here). Thats why I went with this solution.

Note: The tests need authentik-nix input and a small overlay such that pkgs.authenticComponents (overrideScope) is available.

WARNING: Should be merged when upstream nix-community/authentik-nix#156 is merged.

Assited-by: Claude Opus 4.8 only for debugging and design decisions on this module. (Documentation is not super great in authentik)

[gabyx]

@srid: Would be cool if somebody could review this =). Its already used and works well.