Generate `allowed_symlinks.txt` when symlinks are used in module

[Infinidoge]

See the 1.20 Pre-release 7 patch notes and corresponding help article.

The naïve solution would be to do

symlinks = {
  "allowed_symlinks.txt" = pkgs.writeText "allowed_symlinks.txt" "/nix/store";
};

An ideal solution would be

alowedSymlinks = [ "/nix/store" ];

This would solve the problem brought up in #74.

[Misterio77]

This should work too (format from 7408a56), and has all the module-system niceties:

symlinks = {
  "allowed_symlinks.txt" = {
    value = ["/nix/store"];
    format = {
      type = lib.types.listOf str;
      generate = name: value: pkgs.writeText name (lib.concatStringsSep "\n" value);
    };
  };
};

But a convenience allowedSymlinks would be nice.