modify_config_file needs to have the configurations/editor logic

[ReimarBauer]

To ensure consistency, we need to update modify_config_file to use MSUIDefaultConfig.fixed_dict_options and MSUIDefaultConfig.fixed_list_options, mirroring the logic used in the config editor UI.

We can solidify this logic for this module by adding tests.

[ReimarBauer]

MSUIDefaultConfig.fixed_dict_options is already there https://github.com/Open-MSS/MSS/blob/stable/mslib/utils/config.py#L600