Add sharding keys for cluster tables

The following tables have a desired_sharding_key set that needs to be backfilled:

• cluster_providers_aws - #584431
• cluster_providers_gcp - #584432
• clusters_kubernetes_namespaces - #584433
• deployment_clusters - Complete ✅
  • Backfill - !216932 (merged)
  • Finalize backfill - !220574 (merged)
  • Add constraint - !220940 (merged)
• deployment_merge_requests - Complete ✅