Create a /detokenize example server endpoint

[BruceMacD]

Expected Behavior

The example server has a /tokenize endpoint which converts strings to tokens. I am storing the response from this endpoint to calculate the token length of an input (for context window reasons) so I'd also like a /detokenize to convert this back into a string for prompting reasons.

Current Behavior

No /detokenize endpoint.

[BruceMacD]

I can open a PR for this myself.