[Feature]: CMake package and FetchContent installation support

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Problem / use case

libchttpx currently provides Makefile builds, pkg-config metadata, native Linux packages, installer scripts, and Docker images. C/C++ projects that use CMake still need custom include/library discovery or shell installation steps.

Providing a first-class CMake package would make the library easier to consume as both an installed dependency and a source dependency, especially in cross-platform projects and CI.

Proposed solution

Add a maintained CMakeLists.txt and exported CMake package target.

Desired consumer experience after system installation:

find_package(libchttpx CONFIG REQUIRED)
target_link_libraries(my_server PRIVATE libchttpx::libchttpx)

And optionally directly from source:

include(FetchContent)
FetchContent_Declare(libchttpx GIT_REPOSITORY https://github.com/netcorelink/libchttpx.git GIT_TAG vX.Y.Z)
FetchContent_MakeAvailable(libchttpx)
target_link_libraries(my_server PRIVATE libchttpx::libchttpx)

Acceptance criteria

• CMake can build the shared library on supported Linux and Windows configurations.
• cmake --install installs headers, library, and libchttpxConfig.cmake/version metadata.
• Installed package exports libchttpx::libchttpx.
• cJSON and platform dependencies are propagated correctly to consumers.
• FetchContent/add_subdirectory use does not modify global compiler settings unnecessarily.
• Existing Makefile and pkg-config workflows continue to work.
• CI builds a small consumer project using both find_package and FetchContent.
• Native packages include the CMake config files.
• EN/RU installation documentation includes CMake examples.