common: introduce http.h for httplib-based client (#16373)

common: introduce http.h for httplib-based client (#16373)

* common: introduce http.h for httplib-based client

This change moves cpp-httplib based URL parsing and client setup into
a new header `common/http.h`, and integrates it in `arg.cpp` and `run.cpp`.

It is an iteration towards removing libcurl, while intentionally
minimizing changes to existing code to guarantee the same behavior when
`LLAMA_CURL` is used.

Signed-off-by: Adrien Gallouët <redacted>
* tools : add missing WIN32_LEAN_AND_MEAN

Signed-off-by: Adrien Gallouët <redacted>
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Signed-off-by: Adrien Gallouët <redacted>
Signed-off-by: Adrien Gallouët <redacted>