server : (refactoring) do not rely on JSON internally (#10643)

* server : (refactoring) reduce usage of json internally

* move all response types to struct

* wip [no ci]

* many fixes

* add virtual function

* fix index

* minor style fix

* add std::move

* refactor handle_completions_generic

* add virtual functions

* remove server.hpp

* clarify server_sent_event RFC specs

* apply review comments

* fix model_alias and completion_probabilities

* small clean up

* remove virtual for to_json_oai_compat()

* naming oai_compat --> oaicompat

* fix unwanted recursive call

* update docs