func invoke support for Kafka functions

`func invoke` currently only works for HTTP functions. Add support for invoking Kafka functions by producing a test message to the configured topic. The CLI should read `KAFKA_BROKERS` and `KAFKA_TOPICS` from `func.yaml` and send a message.

Blocked by knative-extensions/func-go#176 (producing support — CLI needs to produce messages to a Kafka topic).

Start at the `func invoke` CLI entry point and the `func.yaml` configuration fields `KAFKA_BROKERS` and `KAFKA_TOPICS`. Review the blocked func-go#176 producing support, then verify that invoking a Kafka function sends a test message to the configured topic.

Written by the indexing model from the issue text.