Add cron worker to automatically rollout zoekt exact code search to paid namespaces

What does this MR do and why?

Duo summary:

This merge request introduces a new Zoekt rollout system for search functionality. It adds a planning service to determine how to distribute search indices across available nodes, a provisioning service to execute the planned changes, and a rollout service to orchestrate the entire process. The changes also include a new worker to periodically run the rollout process, along with associated configuration and feature flag settings. Additionally, there are minor adjustments to existing models and scopes to support these new features. Overall, this update aims to improve the management and distribution of search indices across the system's infrastructure.

This is all behind the feature flag zoekt_rollout_worker

References

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• #508836

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