Move task data models where debusine.client can also use them?

!527 (merged) introduces pydantic models for the various kinds of task data, and task data is exposed to debusine client in debusine.client.models.WorkRequestRequest and WorkRequestResponse.

pydantic task data models could then be moved to a new module debusine.client.models.tasks, so that debusine clients can use them to nicely construct valid task data.

This is an architectural change that I'm proposing. If there is agreement about this, I can proceed with the implementation once !527 (merged) has landed in devel

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