CPACK_EXT_ENABLE_STAGING enables optional staging
and CPACK_EXT_PACKAGE_SCRIPT allows to specify an optional
script file that can package staged files via an
external packaging tool.
Issue: #18236
This generator doesn't actually package the files. Instead, it
provides a metadata JSON file that can be used by external packaging
software to do its own packaging. This JSON file provides information
about the components, component groups, installation types, and CMake
projects.