Add support for GCP pub-sub consumer

[rrs45]

Lot of people are waiting on the official rust client for GCP pub-sub. It would really great if you could create a consumer to start with. Thanks

[jrconlin]

Totally agree. However, while I'm the current owner and can do some base maintenance, I've got other, more pressing demands of my time.

Quite happy to review and accept PRs that add that, so if you've got time or can drum up folks who can lend a hand there, 👍🏻 .