Reimplement integration tests using embedded kafka and containerized GCS.

[Claudenw]

Current tests use live backend and so are difficult for non Aiven developers to execute. This change should allow any developer to run the integration tests from their development machine without resorting to a full cloud environment.

[C0urante]

Hard -1 on re-implementing the Kafka layer of these tests. The integration testing library for Kafka Connect already handles bringing up an embedded Kafka cluster and is extremely well-tested and popular. Switching over to containerized Kafka will add runtime bloat and decrease portability.

Adding opt-in or opt-out containerized support for any GCP components is alright, but it should still be possible to run these tests against real BigQuery so as not to lose coverage.

[Claudenw]

My error. I somehow missed the embedded Kafka cluster for testing and was thinking that needed to be implemented.

I agree that the rest of the tests need to go either reald BigQuery or BigQuery testcontainer.