Allow `do_lower_case=True` for any tokenizer
# 🚀 Feature request

Extract the `do_lower_case` option to make it available for any tokenizer. Not just those that initially supported this, like the `BERT` tokenizers.

## Motivation

Sometimes we want to specify `do_lower_case=True` in the `tokenizer_config.json` of a custom tokenizer to activate the lowercasing. The problem is that this obviously works only for tokenizers based on one that originally used this option.

I think we should extract this feature to make it a shared one, that could be used with any tokenizer.

Example of a model that would need this described here: https://github.com/huggingface/transformers/issues/9518

## Special care points

- Make sure the `convert_slow_tokenizer` script also handles this, to activate the option in the resulting fast tokenizer.
- Maybe some other options could have the same treatment?

cc @LysandreJik @sgugger
Discussed offline with @n1t0: our current decision is to wait for https://github.com/huggingface/tokenizers/issues/659 to be resolved before moving on with this issue. 
This is the better tradeoff as the alternative would imply duplicating a lot of logic in `transformers` that's already present but not exposed by `tokenizers`.