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 <angt@huggingface.co>

* tools : add missing WIN32_LEAN_AND_MEAN

Signed-off-by: Adrien Gallouët <adrien@gallouet.fr>

---------

Signed-off-by: Adrien Gallouët <angt@huggingface.co>
Signed-off-by: Adrien Gallouët <adrien@gallouet.fr>

common/CMakeLists.txt

@@ -56,6 +56,7 @@ add_library(${TARGET} STATIC
     common.h
     console.cpp
     console.h
+    http.h
     json-partial.cpp
     json-partial.h
     json-schema-to-grammar.cpp