Kafka functions: structured configuration in func.yaml (optional)

Kafka connection is currently configured via plain environment variables (`KAFKA_BROKERS`, `KAFKA_TOPICS`, `KAFKA_CONSUMER_GROUP`). An alternative would be a structured `middleware.kafka` section in `func.yaml` for brokers, topics, consumer group, and auth settings. The CLI would translate this into environment variables at build/deploy time.

This may not be necessary — env vars work well and are already supported via `func config envs add` with plain values, Secrets, and ConfigMaps. A structured config section would add convenience but also complexity. Needs discussion on whether the value justifies the cost.

Start by reviewing the existing Kafka environment-variable support and how func config envs add handles plain values, Secrets, and ConfigMaps. Discuss whether a middleware.kafka section in func.yaml would justify the added complexity; done means a maintainer decision on whether to pursue the structured configuration.

Written by the indexing model from the issue text.