feat: add kafka connection

[Yisaer]

Summary

Add Kafka named connection support and allow Kafka sink configs to reuse broker connection settings through connectionSelector.

Changes

• Added a stateless Kafka connection implementation.

  • Parses Kafka broker, TLS, and SASL settings.
  • Uses kafka-go dial as the health check.
  • Does not hold producer, consumer, or long-lived Kafka network resources.
  • Registers the kafka connection provider when the Kafka implementation package is loaded.

• Updated Kafka sink metadata.

  • Added connectionSelector.
  • Marked Kafka broker, SASL, and TLS-related fields as connection-related.

• Added test coverage.

  • Unit coverage for Kafka connection provisioning and registration.
  • Config merge coverage for Kafka sink connectionSelector.
  • FVT coverage for memory source -> Kafka sink using a Kafka named connection.

• Added Kafka FVT as a standalone test job in the main test workflow.

  • Starts Redpanda locally.
  • Creates a Kafka named connection.
  • Creates a Kafka sink rule using only connectionSelector + topic.
  • Verifies the message is written to Kafka.

Verification

• go test ./extensions/impl/kafka -run 'TestKafkaConnection'
• go test ./internal/topo/node/conf -run TestOverwriteKafkaConnectionProps
• go test -trimpath -tags='full deadlock' -count=1 -v ./fvt -run TestKafkaConnectionSelectorSinkE2E
• go test -tags full ./internal/binder/io
• python3 -m json.tool extensions/sinks/kafka/kafka.json

Note: The final workflow-only adjustment was pushed without rerunning local tests per maintainer request.