sample kafka avro producer + consumer

[etrandafir93]

Adds two new sample modules — producer_kafka_avro and consumer_kafka_avro —
demonstrating how to use Spring Cloud Contract when Kafka messages are serialized with Avro (Confluent wire format).

The producer publishes a book.returned event encoded as an Avro Book object;
the consumer listens to the same topic, deserializes the message, and triggers an email notification.

Each side comes with two contract testing flavors.

• The JSON flavor expresses the expected message as human-readable fields in the contract:
  • SCC deserializes the Avro bytes back to a Book,
  • converts it to JSON,
  • and asserts field-by-field
  • straightforward failure messages at the cost of two extra serialization round-trips.
• The binary flavor references a pre-serialized .bin fixture
  • (generated once and committed to source control)
  • the contract compares raw bytes on the producer side,
  • the stub runner replays those exact bytes through the consumer's full deserialization stack on the consumer side
  • a stricter wire-format guarantee, but harder-to-read failure output.