Support Sentinel usernames

Currently we support Redis Sentinel passwords (set via requirepass), but Redis Sentinels (and Redis) support ACLs via custom usernames (https://redis.io/docs/latest/operate/oss_and_stack/management/security/acl/).

We currently have global.redis.sentinelAuth that should be extended to provide a Sentinel username. Then we need to update the config files for all the components:

• Workhorse uses SentinelUsername: gitlab-org/gitlab!155209 (merged)
• GitLab Exporter has the redis_sentinel_username config option: gitlab-org/ruby/gems/gitlab-exporter!208 (merged)
• GitLab Rails uses redis-rb and redis-client. This uses the sentinel_username config option (https://github.com/redis-rb/redis-client?tab=readme-ov-file#sentinel-support, https://github.com/redis-rb/redis-client/blob/6d55f61cac62af91aebbd5a1d00eae7a8d940b9e/lib/redis_client/sentinel_config.rb#L12-L32). We may also consider supporting Sentinel URLs to cover TLS support at the same time.
• KAS allows specifying a redis.username that is used in Sentinel. This might be fine as a first iteration, but we might want to support an explicit Sentinel username so people can roll out Sentinel ACLs independently of Redis ACLs: gitlab-org/cluster-integration/gitlab-agent#597 (closed)
• Container Registry supports sentinelusername: gitlab-org/container-registry!1642 (merged)

Other references:

• #2902 (comment 1927848248)
• !3792 (merged)