[RebuildableSetCache] [STEP-1] Add RebuildableSetCache with feature flag switch

Vasilii Iakliushinrequested to merge
duo-edit-20260130-151934 into master

What does this MR do and why?

This is the first step toward implementing the ref cache with rebuild queue mechanism.

Creates Gitlab::Repositories::RebuildableSetCache as a copy of Gitlab::RepositorySetCache. This class will be extended with rebuild queue functionality in subsequent commits.

Also adds the ref_cache_with_rebuild_queue feature flag and updates Repository#redis_set_cache to switch between the two implementations based on the feature flag.

Contributes to #586458

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Related issues

  • Parent issue: #586458
  • Parent epic: gitlab-org&20516
  • Spike MR: !214418 (closed)

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