Enable compression for pubsub library

[lucmult]

As a customer I would like to have communication compressed from pubsub client to pubsub service.

This allows saving on network bandwidth when running it from other Cloud providers for example.

[garrettjonesgoogle]

Fixed in #1645 ; it will be included in the next release.

[ryankillory]

Given that #1645 appears to have been reverted, I was wondering whether this should be reopened.

Thanks!

[georgespalding]

Seeing that it was a year ago someone asked about this, I'd like to bring this issue up again:
We would like to use compression at the Pub/Sub level to avoid doing it at application level.

The pull request and this issue mentions nothing to motivate why it was reverted.
If there is a good reason for not allowing compression it would be nice if you could explain why.

[ostlerc]

Adding this for posterity. the revert commit says grpc doesn't support compression yet.

Compression is not fully supported in gRPC, can't have it in the library
yet.

[georgespalding]

The lack of this functionality does really hurt now that Google Cloud started charging for inter region pubsub egress network usage.