feat(kafka): add franz-go protocol binding

[akme]

Summary

Adds a new Kafka protocol binding based on franz-go to address the feature request in #1233.

This PR introduces a new module at protocol/kafka_franz/v2 with:

• CloudEvents message decoding from kgo.Record
• CloudEvents encoding into kgo.Record
• franz-go based sender / receiver protocol support
• topic override support via cecontext.WithTopic
• message key support via WithMessageKey
• ACK-driven offset commits for protocol-owned franz-go clients

Why

Many Go services already use franz-go as their Kafka client. Without a native binding, users have to maintain custom adapters to integrate franz-go with the CloudEvents SDK.

This adds a first-class franz-go transport alongside the existing Sarama and Confluent Kafka bindings.

Additional changes

• Adds unit tests covering:
  • message encoding / decoding
  • protocol send behavior
  • ACK to commit behavior
  • fetch error handling
• Updates protocol documentation to list the new franz-go Kafka binding

Validation

• env GOCACHE=/tmp/go-build-cache go test ./... in protocol/kafka_franz/v2