Set up "ContextTypeDrawer.OnGUI" to handle prefab overrides and other standard Unity functionality

Fields for context types don’t support some standard Unity functionality, like showing if the value is a prefab override or having a context menu to apply or revert that value. ContextTypeDrawer.OnGUI should contain either EditorGUI.BeginProperty/EditorGUI.EndProperty or using (new EditorGUI.PropertyScope(...)) to enable that functionality.