Add support for generator expressions in add_custom_command OUTPUT file names

With the new moc --collect-json, when doing multi-config builds (debug and release), we will have to create .json file for each configuration e.g core_debug_metatypes.json and core_release_metatypes.json.

CMake currently does not allow to change the names of files created by add_custom_command commands, because it lacks support for specifying generator expressions in the OUTPUT values.

Upstream issue at https://gitlab.kitware.com/cmake/cmake/issues/12877 and initial WIP patch at https://gitlab.kitware.com/cmake/cmake/merge_requests/3865

Main WIP patch at https://gitlab.kitware.com/cmake/cmake/-/merge_requests/5402