Override this property to return a type that inherits from ScriptableObject. This makes the AssetImporterEditor aware of serialized data outside of the Importer.
When returning a type that inherits from ScriptableObject, the AssetImporterEditor creates an instance of this type for each Editor.targets during Inspector initialization..
This should only be used when you want to manage data outside the ScriptedImporter serialization. See the main AssetImporterEditor page for an example.