"Automate" FFmpeg embedding on iOS

6.8.0 added a new cmake function - qt6_add_ios_ffmpeg_libraries - which sets several properties on the target application, resulting in FFmpeg dynamic libraries embedded (and signed) into the application bundle. While this function is documented in the Qt's cmake manual, it requires some changes to pre-existing CMakeLists.txt in an application using QtMultimedia. It makes sense to update Qt's cmake scripts so that such embedding is done 'automatically' without requiring modifications of pre-existing build scripts.