Add zoekt setting to control rollout batch size

What does this MR do and why?

Duo Summary:

This merge request introduces a new configuration option for Zoekt, a code search tool, called "zoekt_rollout_batch_size". This setting allows administrators to control the number of namespaces processed in each batch during the initial indexing process. The default value is set to 32, but it can be adjusted through the application settings interface. The change includes updates to the application settings model, helper methods, and associated tests to support this new feature. Additionally, the Zoekt rollout worker now uses this setting when executing the rollout process, potentially allowing for better control over resource usage during large-scale indexing operations.

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