Provide an event allowing to edit a document before it's adding in a CR

XWiki allows to listen on DocumentSavingEvent which allows to perform changes on the document about to be saved.

We don't have such kind of event in CR allowing to perform changes on the document about to be saved on the CR. This would be needed in plenty of scenario if we want to see in the diff the same changes as the one that will occur when some listeners are triggered.