Find a better way to handle many_many relation updates

[maxime-rainville]

DataObjectEvent doesn't really work with many_many relation updates because it doesn't trigger a write event.

We managed to partially work around this by putting a custom accessor on the many_many relation and firing a custom event. But that's not really a solution that scales.

    public function ProfessionalCategoryPages(int|array $ids = null): Relation
    {
        $proID = $this->ID;
        $callback = function () use ($proID) {
            EventService::singleton()->dispatch(new ProfessionalCategoryPageAssociation($proID));
        };
        $relation = $this->getManyManyComponents('ProfessionalCategoryPages', $ids);

        // If we are working with an unsaved professional, we might get an UnsavedRelationList
        if ($relation instanceof ManyManyList) {
            // These callbacks will be fired when an item is removed or added to the relation
            $relation->addCallbacks()->add($callback);
            $relation->removeCallbacks()->add($callback);
        }

        return $relation;
    }

If our many_many relations were handled as many_many_through, we could just target the pivot DataObject. Maybe that's the ultimate recommended solution.

After a quick look at ManyManyList or RelationList or DataList, I couldn't find any hooks that could be targeted to figure when an object is added or removed.