server : Add option to return token pieces in /tokenize endpoint (#9108)

* server : added with_pieces functionality to /tokenize endpoint

* server : Add tokenize with pieces tests to server.feature

* Handle case if tokenizer splits along utf8 continuation bytes

* Add example of token splitting

* Remove trailing ws

* Fix trailing ws

* Maybe fix ci

* maybe this fix windows ci?

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Co-authored-by: Xuan Son Nguyen <son@huggingface.co>