Handle changes to the schema provided by a field type

Problem/Motivation

When changing the schema provided by a field type (e.g. IntegerItem::schema()) we would have to migrate the storage for all instances of this field. Currently, we do not handle this situation.

We do track the schema that has been previously installed, but only by keeping track of old definitions + generating the old schema from them. But given the new code, the old schema will be generated differently as well.

Proposed resolution

Schema changes to a field type's schema were not officially supported in d7 either, but modules like entity reference shipped with custom updates taking care of it manually. So we need to
a) decide whether we want to officially support this and
b) find a good solution to the problem if we want to.