Track allowed Redis cross-slot operations

What does this MR do and why?

This MR adds the functionality to track allowed cross-slot commands. This functionality is controlled using a feature-flag. To enable this, the MR also refactors lib/gitlab/instrumentation/redis_cluster_validator.rb to return a result hash instead of raising CrossSlotError. The hash containing metadata will allow the instrumentation layer to handle errors and logs/metrics.

Since redis_base.rb is the only user of redis_cluster_validator, there is no behavioural change beyond those 2 classes.

See main issue: gitlab-com/gl-infra/scalability#2005

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