chore: sidekiq per worker alerts in Mimir

For gitlab-com/gl-infra/scalability#2769:

• Adds a new enqueued_jobs SLI to replace the gitlab_background_jobs:queue aggregation sets
• Creates the Sidekiq per worker alerts in Mimir by extracting sidekiq-queue-rules.jsonnet into a library that accepts recording rule registry object and called by Thanos and Mimir with the selective and unified registry respectively.
• Replace gitlab_background_jobs:ops in dashboards with the sli_aggregations: metrics
• Remove the Sidekiq worker aggregation sets and per worker recording rules sidekiq-per-worker-recording-rules.libsonnet.