feat: support optional finding of externals instead of subdirectories

[aminya]

Fixes #5957

This adds an option to allow usage of external targets that are found via find_package and find_path instead of adding them via add_subdirectory. The implementation will fallback to the directories if a target/header cannot be found.

This is cherry-picked from my PR for allowing filament to be built via vcpkg:
microsoft/vcpkg#41916

[poweifeng]

Closing due to this PR having been idle for a while. If you'd like to continue, would you mind first opening a feature request issue and we can have a discussion there to better understand the need?
(related PRs #8267 #8266 #8245)

[aminya]

@poweifeng The PR was ready to be reviewed. There are multiple issues requesting this already. Could you reopen it? I'll make sure all the CI failures are addressed.

[patrikhuber]

Big thumbs-up from me for this PR - it looks like the best solution allowing to keep the modules in add_subdirectory for the people who want and use filament as a submodule/subdirectory, but will also allow a much smoother, better CMake experience for everyone else using find_package and/or a package manager.