Add multi-topic listening for Kafka

[jeremydmiller]

Summary

Closes #2297

• Adds ListenToKafkaTopics() extension method to subscribe to multiple Kafka topics with a single consumer
• KafkaTopicGroup extends KafkaTopic so it integrates seamlessly with the existing BrokerTransport<KafkaTopic> endpoint discovery
• URI pattern uses concatenated topic names: kafka://topic/orders_invoices_shipments
• Supports consumer config overrides, dead letter queues, and all standard listener modes (inline, buffered, durable)
• End-to-end tests verify messages from multiple topics arrive through a single consumer
• Documentation added to the Kafka transport guide

Test plan

• multi_topic_listening.receive_from_multiple_topics_with_single_consumer — verifies messages from two topics are received
• multi_topic_listening.topic_group_uri_uses_concatenated_names — verifies endpoint discovery and URI pattern
• Existing KafkaTransportTests all pass (no regressions)
• Configuration precedence tests pass

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