feat: migration to platformatic-kafka

[Tony133]

This PR migrates @fastify/kafka from node-rdkafka to @platformatic/kafka ( see issue: #158 )

Motivation

node-rdkafka relies on native C++ bindings which complicate cross-platform builds and CI pipelines. @platformatic/kafka is a pure JavaScript implementation with a modern async API, better maintained and aligned with the Fastify ecosystem.

Changes

• lib/producer.js — rewritten using @platformatic/kafka Producer; push() is now fully async
• lib/consumer.js — rewritten using @platformatic/kafka Consumer; supports both flow mode (event-based streaming) and batch mode (consume(n, cbk)) with configurable timeout
• index.js — plugin structure unchanged; wiring updated to match new producer/consumer API
• index.d.ts — types updated to reference @platformatic/kafka
• docker-compose.yml — switched to KRaft mode (no Zookeeper) using confluentinc/cp-kafka:7.6.1
• test/ — full rewrite using node:test; covers producer, consumer, error handling, batch timeout, stream destruction
• README.md — updated usage examples and added migration guide from node-rdkafka
• examples/basic.js — added a working end-to-end example showing producer and consumer setup, topic subscription, flow mode consumption, and graceful shutdown

Thanks to the @platformatic team for creating @platformatic/kafka 🙏

[Tony133]

I have used Kafka very little in my work experience 😅, but I believe the important thing was to start laying the groundwork and try to make this conversion, so if you find any errors or parts that could be improved, please feel free to leave feedback. We could compile a list as a reminder so we can update the code.

[ShogunPanda]

Thanks for considering @platformatic/kafka. A few nits for you but in general it looks fine!