Allow custom string instead of FQCN for object_class discriminator field

[stefgodin]

Feature Request

Currently, in Translatable, the created table contains an object_class column that stores the specific class name.

I propose allowing custom strings (opt-in) instead of specific class names, similar to how Doctrine allows user-defined strings for its inheritance map.

For example, a App\Entity\Skill class is normally mapped to its FQCN, but it could be overridden with a "skill" string.

The reason for this request is that modifying the class FQCN (e.g., moving the namespace or renaming the class) requires running a database migration to remap all translations. Ideally, this change would allow code modification without necessitating a database update.