Make data alter callbacks objects and let them have a configuration form

On second thought, data alter callbacks could use the ability to have configuration forms, too.
Since this is most consistently achieved by making them objects too and proceeding as for services and processes, we should take that approach.

E.g., the "Fulltext field" callback could use some configuration as to which fields to accumulate, instead of just choosing by type. It could also use some configuration as to if and how to respect field boosts.