[Backport 3.1.4] Make AbstractFieldPlugin.onAddRenderService() non-final

I need to extend some field plugins so that in edit mode, they sometimes only get view controls and not edit controls (when it does that is to be decided dynamically). To achieve that, I'd like to override

org.hippoecm.frontend.editor.plugins.field.AbstractFieldPlugin.onAddRenderService(Item<IRenderService>, IRenderService)

but it is declared final for some reason. To my knowledge, this should be a binary compatible change.