Add ClusterRateLimiting for Redis Cluster migration

What does this MR do and why?

This MR adds Gitlab::Redis::ClusterRateLimiting which contains a MultiStore object of primary: ClusterRateLimiting (reads config/redis.cluster_rate_limiting.yml) and secondary: RateLimiting.

Only 2 components use Gitlab::Redis::RateLimiting:

1. Gitlab::ApplicationRateLimiter::BaseStrategy: update with_redis to toggle between 2 stores via feature flag
2. Gitlab::RackAttack::InstrumentedCacheStore via a Rails.cache store: Define RateLimitingMigrator and create RedisCacheStore using it

Why: Scalability team is migrating rate-limiting operations from redis-ratelimiting (redis sentinel) to a Redis Cluster setup.

See issue for more info: gitlab-com/gl-infra/scalability#2036

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