Allow custom subscription name for Pub/Sub healthcheck

[elefeint]

Currently, the Pub/Sub healthcheck attempts a pull from a nonexistent subscription. This has two negative consequences:

1. The pull takes longer
2. Error logs get polluted

Allowing a pull from a known existing subscription would ameliorate issues like #2628 and #2570.

[ThusithaDJ]

Hi @elefeint, I'm aware this bug has been created a couple of days before. But I would like to contribute to the spring community If this not an urgent issue.

[elefeint]

@ThusithaDJ Thank you! I've recreated the issue in our new repo -- GoogleCloudPlatform/spring-cloud-gcp#236 . You'll find more information about what this task entails there.

I will close this issue, and we can continue the conversation on the new issue -- feel free to ask any questions that will help you.