OpenVPN: allow passing credentials with file

[techhazard]

Feedback welcome

Description of changes

This adds an extra option to services.openvpn.servers.<name>.config: authUserPassFile.
When this is specified, it overrides the existing authUserPass option.

When this is specified, the openvpn option auth-users-file is passed with the given filename.

By default the file is not included in the system build (since it's a str not a path: authUserPassFile = "/run/secrets/myfile.txt").

If it is still desirable to include this file in the store, it can be specified as authUserPassFile = "${./myfile.txt}";

TODO: Test this change. I don't know how to do that, since only ever used the default nixpkgs.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• 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/)
• 23.11 Release Notes (or backporting 23.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (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.

[Scrumplex]

If you add an assertion that only one of these options can be set (i.e. cfg.authUserPass -> !cfg.authUserPassFile && cfg.authUserPassFile -> !cfg.authUserPass) you could just add both authUserPassUnsafeOption and authUserPassFileOption to the result, as at least one of them will always be an empty string