Add a built-in Kafka integration and dashboard

[hriszc]

Closes #6026

Summary

• add a new built-in kafka integration with setup docs and icon assets
• ship a Kafka overview dashboard covering broker count, topic partitions, consumer lag, consumer group size, partition offsets, and replicas in sync
• include both standard and _dot dashboard definitions for either metrics mode

Validation

• jq empty pkg/query-service/app/integrations/builtin_integrations/kafka/integration.json pkg/query-service/app/integrations/builtin_integrations/kafka/assets/dashboards/overview.json pkg/query-service/app/integrations/builtin_integrations/kafka/assets/dashboards/overview_dot.json
• git diff --check

Notes

• this environment does not have go, so I could not run the Go build/test pipeline here

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