Add Kind integration test

[TheoBrigitte]

Towards: https://github.com/giantswarm/giantswarm/issues/36445

Here's an addition which add a new CI job to test that a Kafka cluster can be created and is "working" (only test a very simple topic creation and consumption scenario).

Summary

• Adds a GitHub Actions workflow that spins up a Kind cluster, installs the wrapper chart, applies the ephemeral Kafka example, and runs a kafka-topics/produce/consume roundtrip across two brokers (matches TEST_KAFKA.md).
• Adds tests/kind/values.yaml with the minimum wrapper-level toggles needed for Kind (disables VPA, PodMonitor, dashboards — features whose CRDs/webhooks are not in vanilla Kind). Subchart values are intentionally untouched and the install runs without --skip-crds.
• Moves the ephemeral Kafka manifest under examples/kafka-ephemeral/ so it ships alongside the chart as a reusable example.

Test plan

• CI: the new Kind integration test job passes on this PR.
• Operator deployment becomes ready under the strimzi-system namespace.
• kubectl wait kafka/my-cluster --for=condition=Ready succeeds within 10m.
• Producing on my-cluster-broker-1 and consuming on my-cluster-broker-2 returns the test message.

Successful test run https://github.com/giantswarm/strimzi-kafka-operator/actions/runs/25315972276/job/74213462777?pr=39