Add Zoekt::Task model

What does this MR do and why?

This MR adds Search::Zoekt::Task model. It is needed to switch from push to pull-based model for gitlab-zoekt-indexer. This is one of the first steps, which introduces the partitioned zoekt_tasks table. Zoekt indexer nodes are going to pull tasks from it in future MRs.

#430837 (closed)

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