Fixes 18277: Cover multitable inheritance in serialization

[alehaa]

Fixes: #18277

During serialization, custom fields may be available to a model due to multi-table inheritance, but might not be available in serialized data because only direct fields of the model are covered. Now this attribute is only used if available in serialized data. Models using multi-table inheritance must modify their serialize_object() method to cover parent serialization.

Example

Tested example for child models:

class ChildModel(Tenant):
    def serialize_object(self, exclude=None) -> dict[str, Any]:
        data = self.tenant_ptr.serialize_object(exclude)
        data.update(super().serialize_object(exclude))
        return data

If not defined, only fields of the child are serialized.

[jeremystretch]

Thanks @alehaa!