feat(inventory/instances): add option for extraModules to roles

lib/inventory/build-inventory/interface.nix

@@ -410,6 +410,48 @@ in
                       default = { };
                       type = clanLib.types.uniqueDeferredSerializableModule;
                     };
+                    extraModules = lib.mkOption {
+                      description = ''
+                        List of additionally imported `.nix` expressions.
+
+                        Supported types:
+
+                        - **Strings**: Interpreted relative to the 'directory' passed to buildClan.
+                        - **Paths**: should be relative to the current file.
+                        - **Any**: Nix expression must be serializable to JSON.
+
+                        !!! Note
+                            **The import only happens if the machine is part of the service or role.**
+
+                        Other types are passed through to the nixos configuration.
+
+                        ???+ Example
+                            To import the `special.nix` file
+
+                            ```
+                            . Clan Directory
+                            ├── flake.nix
+                            ...
+                            └── modules
+                                ├── special.nix
+                                └── ...
+                            ```
+
+                            ```nix
+                            {
+                              extraModules = [ "modules/special.nix" ];
+                            }
+                            ```
+                      '';
+                      apply = value: if lib.isString value then value else builtins.seq (builtins.toJSON value) value;
+                      default = [ ];
+                      type = types.listOf (
+                        types.oneOf [
+                          types.str
+                          types.anything
+                        ]
+                      );
+                    };
                   };
                 }
               );