Zoekt::Index state pending -> initializing

What does this MR do and why?

Add a new event InitialIndexingEvent which gets published from the initial_indexing task for each pending index. This event is subscribed to the worker Search::Zoekt::InitialIndexingEventWorker which will call Search::Zoekt::IndexingTaskService for each project inside the indexed namespace. And finally, move the index to initializing. Also added a condition that if the feature flag zoekt_initial_indexing_task is enabled then don't call the index in the Search::Zoekt::Index model callback. The indexing should be happened via that scheduling task.

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Related to #482686 (closed)