Add Zoekt private repository FF

What does this MR do and why?

This MR guards the changes introduced by !123151 (merged) behind a second feature flag to allow us to roll the new indexer and private repository support separately.

Screenshots or screen recordings

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Before	After

How to set up and validate locally

You can follow !123151 (merged), but with the addition of Feature.enable(:zoekt_index_private_repositories)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #389741 (closed)