Pass fetch_content_args to FetchContent_Declare for LibXml2

[Manish7093]

Description

fetch_content_args includes FIND_PACKAGE_ARGS, but it is never forwarded to FetchContent_Declare(), as a result, on CMake >= 3.24, find_package(LibXml2) is never attempted and the build always falls back to cloning libxml2 from git.

This also breaks FORCE_SYSTEM_LIBXML2, since CMAKE_REQUIRE_FIND_PACKAGE_LibXml2 only takes effect when FIND_PACKAGE_ARGS is passed to FetchContent_Declare().
In offline build environments (e.g. distro build systems such as Fedora Koji, which
have no network and no git in the buildroot), configuration fails because CMake tries to clone libxml2 instead of using the installed system library.

CMake Error at /usr/share/cmake/Modules/ExternalProject/shared_internal_commands.cmake:928 (message): error: could not find git for clone of libxml2-populate

Note: ${fetch_content_args} is appended as the last argument intentionally bcz everything after the FIND_PACKAGE_ARGS keyword is forwarded to find_package(), so it must come after the other options.

With this change:
On CMake >= 3.24, the system libxml2 is preferred when available, with Git fallback otherwise.

• FORCE_SYSTEM_LIBXML2 correctly requires the system library.
• FORCE_BUILD_LIBXML2 continues to always build from source.

Checklist:

• [X ] I have followed the Contribution Guidelines
• I have added test code and data to prove that my code functions correctly
• I have verified that new and existing tests pass locally with my changes
• [X ] I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation