Add Gitlab::Redis::DbLoadBalancing Redis instance

What does this MR do and why?

Add Gitlab::Redis::DbLoadBalancing Redis instance

This instance will be used to separate Database Load Balancing (see https://docs.gitlab.com/ee/administration/postgresql/database_load_balancing.html) from the Gitlab::Redis::SharedState instance

Part of gitlab-com/gl-infra/scalability#2149 (closed)

Screenshots or screen recordings

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.