Make it possible not to manage merge on ContributionFragmentRegistry implementations

ContributionFragmentRegistry implementations support hot reload because they will handle adding/removal/merge of contributions for complex cases (like removing the 2nd, not last, override of a given contribution, and return the merged result when getting the given contribution from the service).

But sometimes the service does not handle merge, it only needs to get the accurate implementation, also when removing one of the overrides

=> add a hook where you can specify that the contribution registry will not perform any merge, so that #clone and #merge method will not be called.