Feature Request: allow to omit the call to add_subdirectory() in FetchContent_MakeAvailable

Hello

This is is a follow up to: #26199 (closed)

Because of the quite awkward way (see linked ticket) to avoid calling add_subdirectory() from within FetchContent_MakeAvailable(), it would be nice if a key word argument could prevent that.

This use case becomes more prevalent because of the deprecation of the declarative form of FetchContent_Populate() since CMake v3.30.

Reasons for not using add_subdirectory() in 3rd party dependencies are many fold: e.g. unfriendly behaving (old type) of CMakeLists.txt files, projects with poorly supported CMakeLists.txt files because the primary build system is a different one, etc.