Add 'imports' sidekiq shard

• New shard will take only one worker repository_import
  • This currently runs on the catchall shard
  • It'll be removed in a future commit
• Placing this shard on the existing catchall shard as we appear to have plenty of available RAM on those nodes
• Limiting concurrency to 1 to limit the blast radius when Pods are rotated for any reason
• Locking scale to prevent auto-scale from unnecessarily removing in process jobs
• Using the same CPU resource requests and limits as our catchall shard
  • As this workload is not considered urgent these values shall suffice
• Doubling the RAM resources and requests due to seeing occasional OOM events on the catchall shard
• It is estimated, that over a 7 day period of time, we should see a job saturation of 41%
  • Reference: gitlab-org/gitlab#332616 (comment 598446072)
• For staging, we lower the memory requirements and the amount of replicas, because staging is slightly smaller

Addresses: gitlab-org/gitlab#332616 (closed)

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