feat: Add endpoints for tokenize, detokenize and count tokens (#1136)

* Add endpoint to count tokens

* Add tokenize and detokenize endpoints

* Change response key to tokens for tokenize endpoint

* Fix dependency bug

* Cleanup

* Remove example added by mistake

* Move tokenize, detokenize, and count to Extras namespace. Tag existing endpoints

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Co-authored-by: Andrei Betlen <abetlen@gmail.com>