RFC for now, save the special_tokens_mapping

[ArthurZucker]

The motivation behind this is:

• that as of today, without this info you can't do a génération loop because you need to obtain the info from somewhere else.
• ideally the rust crate should be able to provide everything a tokenizer "has" and a mapping from standard names is kinda missing.
• you have to download and read the tokenizer_config.json generated by transformers when it should be fairly easy to keep track of that info!
• transformers v5 refactors its tokenization backend to be minimal, and this is aligned with removing the tokenizer_config.json