func cluster create: optional --kafka to install a Kafka broker

[gauron99]

Problem

Kafka functions (run.kafka) and KEDA consumer-lag scaling (#4051) have no automated tests against a real broker. func cluster create / hack/cluster.sh install no Kafka, so #3956 can't run in CI, and manual testing needs hand-written manifests.

Proposal

Add --kafka ($FUNC_CLUSTER_KAFKA, default off) next to --serving, --eventing, --keda and --tekton:

• install a single-node Kafka broker in a kafka namespace, reachable at kafka.kafka.svc.cluster.local:9092
• pin its version in hack/component-versions.json
• optionally a SASL_PLAINTEXT listener with a fixed test user, to test credentials (TriggerAuthentication)

Open questions

• Plain apache/kafka in KRaft mode (a single Deployment and Service; small and fast, and it worked for manual testing of Add KEDA Kafka consumer-lag scaling support #4051) or Strimzi (as Kafka functions: integration and E2E tests #3956 suggests; closer to production, heavier on CI runners)?
• SASL/TLS listeners in the default install or behind a separate flag?

Related: #3956, #4051