Summary:
Added test stream functionality. When a stream is configured, it will try to
consume messages from a kafka topic and return them back to the user.
For now, the messages aren't transformed, so it just returns the payload string.
Depends on D1466
Next steps are persisting stream metadata and transforming messages in order to
store them in the graph.
Reviewers: teon.banek, mtomic
Reviewed By: teon.banek
Subscribers: pullbot, buda
Differential Revision: https://phabricator.memgraph.io/D1474
a026c4c764