Add getOrderingKey in org.apache.beam.sdk.io.gcp.pubsub.PubsubMessage

[damccorm]

Currently ordering key is added in python sdk in PR: #14767

Need to implement similarly for java (org.apache.beam.sdk.io.gcp.pubsub.PubsubMessage).

Is this in the roadmap? If yes, could you ping the Jira here? 

Thanks!

 

Imported from Jira BEAM-13592. Original Jira may contain additional context.
Reported by: kedia56.

[egalpin]

relates to #21162

[egalpin]

Fixed by #22216