server : add self-extend support (#5104)

* Ported self extension to server example

* Update server.cpp

* Fixed prompt caching without self extend

* Update server.cpp

* Added description to server readme.

* Update server.cpp

* Update server.cpp

* Update server.cpp

* Update server.cpp

* Update README.md

* Changed descriptions

* server : formatting

* Update examples/server/server.cpp

Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>

* Update examples/server/server.cpp

Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>

* Update server.cpp

* Update server.cpp

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Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>