server: split HTTP into its own interface (#17216)

server: split HTTP into its own interface (#17216)

* server: split HTTP into its own interface

* move server-http and httplib to its own file

* add the remaining endpoints

* fix exception/error handling

* renaming

* missing header

* fix missing windows header

* fix error responses from http layer

* fix slot save/restore handler

* fix case where only one stream chunk is returned

* add NOMINMAX

* do not call sink.write on empty data

* use safe_json_to_str for SSE

* clean up

* add some comments

* improve usage of next()

* bring back the "server is listening on" message

* more generic handler

* add req.headers

* move the chat template print to init()

* add req.path

* cont : minor

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Co-authored-by: Georgi Gerganov <redacted>