Profile field types need an enable/disable mechanism

If an extension defines a new Profile Field type, it needs some kind of mechanism to enable, disable and purge data of this particular Profile Field type, to avoid cases of left profile fields based on no-longer existing profile field types that break the forum completely.

A mechanism similar to the one of notifications, adding enable, disable and purge methods to the profile field type manager class, that would be called from the ext class of the extension, seem like the most appropriate way forward.

PS: What is the correct component for this ticket?