feat(sink): add Kafka sink

[psteinroe]

Summary

• Add KafkaSink that publishes events to Kafka topics
• Sends event.payload as JSON message value
• Supports dynamic topic and partition key via event metadata

Configuration

sink:
  type: kafka
  brokers: localhost:9092
  topic: events        # optional default topic

Routing

Topic and key are resolved from:

1. topic and partition_key fields in event metadata
2. Fallback to config values