Set message ordering key for Google Pubsub

[damccorm]

Message ordering is currently not supported on the producer side.

https://github.com/apache/beam/blob/master/sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/pubsub/PubsubMessage.java#L44

 

Imported from Jira BEAM-12007. Original Jira may contain additional context.
Reported by: victuos.