Add an integration test for the Kafka output

[robbavey]

Add a basic integration test for the shipper Kafka output. The test should:

• Start Kafka as a Docker container.
• Start the shipper binary with a Kafka output configured to write to the Kafka Docker container.
• Setup topics as the target for the events that will be ingested during the test.
• Publish events to the shipper via the gRPC API and ensure they are ingested into the target topics with the expected format.

[cmacknz]

Relates to #148, the test setup between the two is likely to be extremely similar.