Replace Docker-based integration tests with embedded integration tests

[C0urante]

When we wrote the Docker-based integration tests, there were no options for running an embedded instance of the Connect framework in a unit test. Now, there's the EmbeddedConnectCluster class provided and supported by Kafka that we can leverage for integration tests, and Confluent also provides an option for bringing up an embedded instance of Schema Registry.

Using both of these, it should be possible to recreate the existing tests very closely but with an improved workflow for running and expanding them that doesn't require shell scripts or Docker image building, and also enables more flexibility in the test cases covered. We might build on this to add specific tests for upsert/delete if/when that's added to the connector.