nixos/pangolin: refactor

[SigmaSquadron]

• Increased compliance with RFC 0042;
• Proper modules for Gerbil and Badger;
  • WIP: we do not yet know if Gerbil can be ran as a standalone server, and which parts of Traefik, if any, would be necessary to accomplish this.
• a lot of abstraction for previously reused code;
• Dehardcoded values for Gerbil's CLI;
  • WIP: The actual interface for 'config.services.gerbil.settings'.
• WIP: Redesigned Traefik config for multiple DNS providers and multiple domains;
• Easier way to configure ports for raw TCP/UDP resources;
• Gerbil runs as a DynamicUser;
• Removed options that duplicated the functionality from the ACME or Traefik modules;
• Added configurable user/group options.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• Nixpkgs 25.11 Release Notes (or backporting 25.05 Nixpkgs Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
• NixOS 25.11 Release Notes (or backporting 25.05 NixOS Release notes)
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md, pkgs/README.md, maintainers/README.md and other contributing documentation in corresponding paths.

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