Add option to generate load via Google PubSub

See #279 (closed) for a similar implementation for HTTP requests.

changed milestone to %0.7.0

added modul/benchmarks/load-generator label

assigned to @she

We might also consider using PubSub Lite instead or in addition to PubSub, as it features a Kafka-compatible API.

mentioned in merge request !225 (merged)

mentioned in issue #336 (closed)

mentioned in commit f95b7b7a

closed with merge request !225 (merged)