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

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[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!