Cannot deserialize notification channel data without a database lookup

(see !587 (comment 464887))

In the pydantic models for channel data we don't have a tag in the data type or in the TaskNotification fields that tells what kind of data is expected. This makes it potentially impossible to validate or deserialize a task without looking up the channel type by channel name in the database. Is this something we want to change?

https://docs.pydantic.dev/1.10/usage/types/#discriminated-unions-aka-tagged-unions could be a way to do this, and a probably easier way would be to add the method field to TaskNotification, with the same value as in debusine.db.NotificationChannel