Prepare clusters_kubernetes_namespaces for sharding

What does this MR do and why?

Prepare clusters_kubernetes_namespaces for sharding:

• add organization_id, group_id, and sharding_project_id columns
• create a trigger that assigns the sharding keys from the associated cluster (copies organization_id, group_id, and project_id from the clusters table)
• create indexes on each sharding key column (organization_id, group_id, sharding_project_id)
• add foreign key constraints on organization_id (to organizations), group_id (to namespaces), and sharding_project_id (to projects)
• add a multi-column not null constraint (not valid) ensuring exactly one sharding key is set
• queue a batched background migration to backfill the sharding key columns from the associated cluster

Addresses #584433

Changelog: other

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