chore(pubsub): Add sample for subscribe avro records with revisions

[quartzmo]

Add sample for pubsub_subscribe_avro_records_with_revisions.

The design is based on SubscribeWithAvroSchemaRevisionsExample.java:

googleapis/java-pubsub#1469

[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• google-cloud-pubsub/samples/pubsub_subscribe_avro_records_with_revisions.rb:18, tag pubsub_subscribe_avro_records_with_revisions

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[aandreassa]

Just a bunch of nits, but it looks good to go!

For new samples, we always strive for code readability over anything else. So that would something like no long lines, no nested arguments (within reason), descriptive comments. You got it though, LGTM!

[quartzmo]

@aandreassa Ruby 3.1 is no longer supported, correct? Is it OK to merge this PR despite the failing 3.1 check?