common: introduce http.h for httplib-based client

[angt]

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.

[ggerganov]

I think this is the only relevant error that needs to be resolved before merge: https://github.com/ggml-org/llama.cpp/actions/runs/18166867333/job/51711390130?pr=16373#step:5:4676

[angt]

I only fixed run.cpp.
Here the list of files with the same "issue":

$ grep -rl "<windows.h>" . | xargs grep -L "WIN32_LEAN_AND_MEAN"
./tools/gguf-split/gguf-split.cpp
./tools/rpc/rpc-server.cpp
./ggml/src/ggml-cpu/kleidiai/kleidiai.cpp
./ggml/src/ggml-sycl/dpct/helper.hpp
./ggml/src/ggml-vulkan/ggml-vulkan.cpp
./ggml/src/ggml-vulkan/vulkan-shaders/vulkan-shaders-gen.cpp
./common/log.cpp

I think we should use a common header to cleanup everything.