Kafka middleware: pluggable serialization (Avro, Protobuf, Schema Registry)

Add configurable message deserialization so the handler can receive typed objects instead of raw bytes:

- JSON to typed struct
- Avro with Confluent Schema Registry
- Protobuf with schema registry
- Custom deserializers

Related: #180 (CloudEvent detection — both extend the message format).

Start by locating the Kafka middleware and the handler path that currently exposes raw bytes, then review related issue #180 for interaction with message formats. Done means the handler can receive typed objects through JSON, Avro with Confluent Schema Registry, Protobuf with schema registry, and custom deserializers.

Written by the indexing model from the issue text.