chore: Add pubsub rollback schema sample

[torreypayne]

Adds new samples and acceptance tests for Pub/Sub schema evolution features, mirroring the equivalent changes in Java (googleapis/java-pubsub#1469).

[snippet-bot]

Here is the summary of changes.

You are about to add 2 region tags.

• google-cloud-pubsub/samples/pubsub_rollback_schema.rb:15, tag pubsub_rollback_schema
• google-cloud-pubsub/samples/pubsub_update_topic_schema.rb:15, tag pubsub_update_topic_schema

---

This comment is generated by snippet-bot.
If you find problems with this result, please file an issue at:
https://github.com/googleapis/repo-automation-bots/issues.
To update this comment, add snippet-bot:force-run label or use the checkbox below:

• Refresh this comment

[aandreassa]

It looks great! Can you please make sure the tests passes? (unrelated, but I wonder if it is just a flaky thing).

You can retry by adding kokoro:force-run label to this PR and it will update eventually.

[torreypayne]

It looks great! Can you please make sure the tests passes? (unrelated, but I wonder if it is just a flaky thing).

You can retry by adding kokoro:force-run label to this PR and it will update eventually.

@aandreassa I landed the fix to the flaky tests and rebased; builds are green!

[aandreassa]

Woo! LGTM

[aandreassa]

You should see a similar case in this PR when merging (i.e. difference in title vs commit text).

We do chore for samples as they don't need an actual publication to Rubygems.

[torreypayne]

You should see a similar case in this PR when merging (i.e. difference in title vs commit text).

We do chore for samples as they don't need an actual publication to Rubygems.

Updated the commit message to be chore(...)! Going to merge