feat: export_header 增加 items的选项

lib/tallty_import_export/attr/export_header.rb

 module TalltyImportExport
   module Attr
-    class ExportHeader
+    class ExportHeaderItem
       include AttrJson::Model
       attr_json_config(bad_cast: :as_nil, unknown_key: :allow)
 
@@ -17,5 +17,12 @@ module TalltyImportExport
       attr_json :proc, ActiveModel::Type::Value.new, array: true
       attr_json :list, self.to_type, array: true
     end
+
+    class ExportHeader
+      include AttrJson::Model
+      attr_json_config(bad_cast: :as_nil, unknown_key: :allow)
+
+      attr_json :items, ExportHeaderItem.to_type, array: true
+    end
   end
 end