[`TokenizationUtils`] add support for `split_special_tokens`

[ArthurZucker]

What does this PR do?

Adds support for split_special_tokens for fast models as well

• [ ] deprecate split_special_tokens for encode_special_tokens for API consistency
• [ ] make sure this is saved and used not only as kwargs but also the attribute
• [ ] add some tests
• [ ] add some docs

[ArthurZucker]

Wip, not urgent! Can already be done but it's not save / loaded