server : OAI API compatibility (#4198)

server : OAI API compatibility (#4198)

* Add openai-compatible POST /v1/chat/completions API endpoint to server example

* fix code style

* Update server README.md

* Improve server README.md

* Fix server.cpp code style according to review

* server : some style changes

* server : indentation

* server : enable special tokens during tokenization by default

* server : minor code style

* server : change random string generator

* straightforward /v1/models endpoint

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Co-authored-by: kir-gadjello <redacted>
Co-authored-by: Tobi Lütke <redacted>