Add ClusterSessions for redis sessions migration

What does this MR do and why?

This MR adds a new Redis class ClusterSessions which is going to be the new Redis Cluster store for the existing Sessions Redis store. Migration is done by wrapping the Sessions class with Multistore, which handles dual writes to both stores via FF toggles.

This is the 3rd MR in the series of MR in gitlab-com/gl-infra/data-access/durability/team#35 (comment 2278902354).

❗ ❗ BIG WARNING: Ensure use_primary_store_as_default_for_sessions FF is disabled (currently enabled from seemingly previous migration), otherwise traffic would start going into the ClusterSession.

References

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• gitlab-com/gl-infra/data-access/durability/team#28

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