Add ClusterRepositoryCache class for migration

What does this MR do and why?

This MR adds ClusterRepositoryCache class for migrating repository cache workload from redis-repository-cache (a standalone Redis with sentinels) to a Redis Cluster for gitlab.com.

Gitlab::Redis::ClusterRepositoryCache will connect to a Redis Cluster for .com deployment and help migrate repository cache workload from Gitlab::Redis::RepositoryCache.

See issue: gitlab-com/gl-infra/scalability#2854 (closed)

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Before	After

How to set up and validate locally

1. For SM users without an external Redis for repository cache, i.e. no config/redis.repository_cache.yml, we can verify that this change does not affect them at all by

rm config/redis.repository_cache.yml # use gdk reconfigure to restore
gdk rails c

2. The 2 stores Cache and RepositoryCache should read/write to the same store.

[1] pry(main)> Gitlab::Redis::Cache.params
=> {:cluster=>["redis://localhost:6001"], :instrumentation_class=>Gitlab::Instrumentation::Redis::Cache, :db=>0}
[2] pry(main)> Gitlab::Redis::RepositoryCache.params
=> {:cluster=>["redis://localhost:6001"], :instrumentation_class=>Gitlab::Instrumentation::Redis::RepositoryCache, :db=>0}
[6] pry(main)> Gitlab::Redis::RepositoryCache.with {|c| c.set('testrepocache', '12345') }
=> "OK"
[7] pry(main)> Gitlab::Redis::Cache.with {|c| c.get('testrepocache') }

3. ClusterRepositoryCache should have the same params and read the same value, i.e., if only config/redis.cache.yml is defined, all 3 stores are effectively identical.

[8] pry(main)> Gitlab::Redis::ClusterRepositoryCache.with {|c| c.get('testrepocache') }
=> "12345"
[9] pry(main)> Gitlab::Redis::ClusterRepositoryCache.params
=> {:cluster=>["redis://localhost:6001"], :instrumentation_class=>Gitlab::Instrumentation::Redis::ClusterRepositoryCache, :db=>0}

4. The MultiStore does not affect SM users too since it defaults to the secondary store if same_redis_store? is true.

[11] pry(main)> Gitlab::Redis::RepositoryCache.multistore.send(:same_redis_store?)
=> true

For gitlab.com:

We have config/redis.cache.yml and config/redis.repository_cache.yml configured. Since we have not provisioned a Cluster for ClusterRepositoryCache yet, the missing configuration will mean it uses Cache's details. However, as long as the feature-flag is disabled, it will not be used.

1. Run gdk reconfigure and update config/redis.repository_cache.yml to use a different db from cache

  gitlab git:(sc1-migrate-repository-cache) cat config/redis.repository_cache.yml
---
development: unix:/Users/sylvesterchin/work/gitlab-development-kit/redis/redis.socket?db=3
test: unix:/Users/sylvesterchin/work/gitlab-development-kit/redis/redis.socket?db=12

2. Verify using gdk rails c that RepositoryCache has a different configuration:

[1] pry(main)> Gitlab::Redis::RepositoryCache.params
=> {:instrumentation_class=>Gitlab::Instrumentation::Redis::RepositoryCache, :path=>"/Users/sylvesterchin/work/gitlab-development-kit/redis/redis.socket", :db=>3}
[2] pry(main)> Gitlab::Redis::ClusterRepositoryCache.params
=> {:instrumentation_class=>Gitlab::Instrumentation::Redis::ClusterRepositoryCache, :path=>"/Users/sylvesterchin/work/gitlab-development-kit/redis/redis.socket", :db=>2}
[3] pry(main)> Gitlab::Redis::Cache.params
=> {:instrumentation_class=>Gitlab::Instrumentation::Redis::Cache, :path=>"/Users/sylvesterchin/work/gitlab-development-kit/redis/redis.socket", :db=>2}

3. Verify that RepositoryCache writes to a different Redis db (with both feature-flags disabled), i.e., config fallback behaviour is working fine.

[4] pry(main)> Gitlab::Redis::RepositoryCache.with {|c| c.set('testrepocache', 'abcdef') }
  Feature::FlipperGate Pluck (3.1ms)  SELECT "feature_gates"."key", "feature_gates"."value" FROM "feature_gates" WHERE "feature_gates"."feature_key" = 'use_primary_and_secondary_stores_for_repository_cache' /*application:console,db_config_name:main,console_hostname:SylvestersMBP2.localdomain,console_username:sylvesterchin,line:/lib/feature.rb:314:in `block in current_feature_value'*/
  Feature::FlipperGate Pluck (0.3ms)  SELECT "feature_gates"."key", "feature_gates"."value" FROM "feature_gates" WHERE "feature_gates"."feature_key" = 'use_primary_store_as_default_for_repository_cache' /*application:console,db_config_name:main,console_hostname:SylvestersMBP2.localdomain,console_username:sylvesterchin,line:/lib/feature.rb:314:in `block in current_feature_value'*/
=> "OK"
[5] pry(main)> Gitlab::Redis::ClusterRepositoryCache.with {|c| c.get('testrepocache') }
=> nil
[6] pry(main)> Gitlab::Redis::Cache.with {|c| c.get('testrepocache') }
=> nil