Optionally allow gitlab.database.loadBalancing for Sidekiq

Summary

With gitlab-org/gitlab!55881 (merged) we will soon be able to take advantage of database loadbalancing for Sidekiq.

We currently support connecting to the primary in charts https://gitlab.com/gitlab-org/charts/gitlab/-/blob/a8ba8bcf12568777a3e7682852f4b6c02e4e3719/charts/gitlab/charts/sidekiq/templates/configmap.yaml#L13-14 , to take advantage of the replicas we will want to add an optional flag to use gitlab.database.loadBalancing instead.