Redis password encryption for split Redis cluster

Proposal

Develop and document Redis password encryption for split Redis cluster

Currently a lot of work has been done on Redis password encryption

• Encrypt redis_password in the rails codebase
• Support encrypted passwords for features that utilize Redis
• Encrypt all Rails passwords stored in gitlab.rb

But currently we don't have a way to do it for split Redis cluster.

Customer requested this feature, and we need develop and document the approach.

More details on customer request on https://gitlab.com/gitlab-com/request-for-help/-/issues/3597 (internal)