在重新加载所有 XR SDK Provider 打包的子系统前，从 SubsystemManager 调用。
When the Editor starts or when packages are installed or removed, the SubsystemManager searches the packages and loads all XR SDK packages that it finds. Handling this event allows you to do work you might need after the subsystem manager loads and initializes new subsystem descriptors.
See Also: SubsystemManager, SubsystemDescriptor.