Add support for DjangoTask's delay_on_commit and apply_async_on_commit methods

[federicobond]

Celery 5.4 introduced some shortcuts for Django projects using a custom task class. This new DjangoTask base class adds the following methods:

• delay_on_commit
• apply_async_on_commit

https://docs.celeryq.dev/en/stable/django/first-steps-with-django.html#trigger-tasks-at-the-end-of-the-database-transaction

Not sure what's the best way to support app.task/shared_task returning instances of this base class but we can start by adding stubs for celery.contrib.django.task.DjangoTask (see API reference).