feat: add support for vLLM Mistral Tokenizer

[gcalmettes]

fix #141

The vllm MistralTokenizer class is a wrapper over the mistral tokenizers from mistral-common.

When the build_vllm_token_enforcer_tokenizer_data function is called, the underlying mistral-common tokenizer is passed instead of the MistralTokenizer, which has the proper methods used by lm-format-enforcer to access the tokens and special tokens of the tokenizer.

This MR adds a check to directly pass the vllm MistralTokenizer tokenizer instead of the underlying mistral-common tokenizer when building the tokenizer data.

[noamgat]

Thanks for the contribution! I will review it this weekend and get back to you

[noamgat]

Merged, thanks!