[WIP] POC: Add Kafka middleware package

[aliok]

NOTE: THIS IS A POC. Do not review the whole thing, but review the overall structure and the integration points.

Add a new kafka/ package that implements a Functions Kafka middleware,
following the same patterns as the existing http/ and cloudevents/ packages.

The middleware runs a sarama consumer group alongside an HTTP health server,
delivering raw messages (key, value, headers) to the user's Handle method.
Offset is committed on success; errors are logged and skipped (for the initial impl's sake).

Example usage is in cmd/fkafka/

Delivery semantics:

• At-least-once, in-order per partition
• No retry or deduplication yet (tracked as TODOs)

This work is part of the ongoing effort to support Kafka in the Functions. Related: knative/func#3923

[knative-prow]

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