Support Redis Sentinel username config option
Currently the Redis username is used to populate the Sentinel username (https://gitlab.com/gitlab-org/cluster-integration/gitlab-agent/-/blob/d11dc332993baca56eec87e224ba3771eb431ff2/cmd/kas/kasapp/configured_app.go#L671).
We probably want to provide an explicit Sentinel username option in case admins want to roll out Sentinel ACLs independently of Redis ACLs.