Move task notifications from jsonschema to pydantic

If I read the code correctly, BaseTask.configure currently removes the notifications key from the task data, validates it with jsonschema and discards it. notifications is however documented as a key for task data for all tasks: https://freexian-team.pages.debian.net/debusine/reference/tasks.html#ontology-for-generic-tasks

Now that we have pydantic data models we can add a new notifications field to BaseTaskData, and have it cleanly validate with all the rest.

I tried this (MR coming soon): various tests fail because the serialized task data structures gain a new field notifications: None, and I'm not sure what is the desired behaviour.

Can we make it official that notifications is part of task data, and absorb the notification definition into BaseTaskData?