WIP [`split_special_tokens`] Add support for `split_special_tokens` argument to encode
# What does this PR do?
Argument name is totally debatable. Will also require a pull request in `tokenizers`. 
The goal is to be able to simply activate and de-activate the special token splitting. Feature was asked in #22490, and is required for some production type cases, where users pass inputs and we don't want them to be able to hack them
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_25081). All of your documentation changes will be reflected on that endpoint.