Enhancement: Explore Polymorphism support for services and repositories

[cofin]

Would it be possible to add Polymorphism support for services and repositories? Currently, I've only been able to make Polymorphism work by creating a service for each subclass. Ideally it would work something like this:

AllAlerts = with_polymorphic(Alert, (UnexpectedAlert, ExpectedAlert))

class AlertService(SQLAlchemyAsyncRepositoryService[AllAlerts]):

    class Repo(SQLAlchemyAsyncRepository[AllAlerts]):
        model_type = AllAlerts

    repository_type = Repo

AllAlerts is an AliasedClass that contains information about the Polymorphic relation. Just like SQLAlchemy, Advanced Alchemy typing would hint that it's methods return the abstract class.

alert: Alert = await alert_service.create(data)

But in practice, it really returns the full Polymorphic identity. I'm not sure how much additional work would be required, but this would be a great boon for services that provide many polymorphic identities.

Originally posted by @njz-cvm in #611 (comment)