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 https://github.com/knative-extensions/func-go/issues/176 (producing support — CLI needs to produce messages to a Kafka topic).

Start with the `func invoke` CLI path and inspect how it reads `func.yaml`; review the producing-support dependency in func-go issue 176 first. Done means `func invoke` can read `KAFKA_BROKERS` and `KAFKA_TOPICS` and send a test message to the configured Kafka topic.

Written by the indexing model from the issue text.