Support @HippoEssentialsGenerated allowsModification=false at class level

In a content bean class, methods annotated with @HippoEssentialsGenerated with parameter allowModifications=false are ignored (i.e. not overwritten) by the BeanWriter (see ESSENTIALS-978). This works well.

The @HippoEssentialsGenerated can be use at class level too, but the allowModifications parameter is ignored at that level. It would be convenient and consistent to support the allowModifications parameter at class level too, so that an entire content bean class is simply ignored by the BeanWriter.