feat(pubsub/v2): add single message transform samples

[hongalex]

Migrates the existing v1 SMT samples to use the v2 library.

[snippet-bot]

Here is the summary of changes.

You are about to add 2 region tags.

• pubsub/subscriptions/create_message_transforms.go:17, tag pubsub_create_subscription_with_smt
• pubsub/topics/create_topic_message_transforms.go:17, tag pubsub_create_topic_with_smt

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[gemini-code-assist]

Summary of Changes

Hello @hongalex, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request adds new Go code samples for Google Cloud Pub/Sub, specifically demonstrating the use of Single Message Transforms (SMT). The samples illustrate how to configure SMTs on both topics and subscriptions, providing a concrete example of data redaction using a JavaScript UDF, and are integrated with system tests to validate their functionality.

Highlights

• New Samples for Single Message Transforms (SMT): Introduced new Go code samples demonstrating how to apply Single Message Transforms (SMT) to Google Cloud Pub/Sub topics and subscriptions.
• v2 Client Library Adoption: The new samples are built using the cloud.google.com/go/pubsub/v2 client library, aligning with the latest API versions.
• JavaScript UDF Example: The SMT examples showcase a practical use case: redacting sensitive data (SSN) from messages using an inline JavaScript User-Defined Function (UDF).
• System Tests: Accompanying system tests have been added for both topic and subscription SMT creation to ensure the samples function correctly.

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[gemini-code-assist]

Code Review

This pull request adds samples for creating Pub/Sub topics and subscriptions with Single Message Transforms (SMT). The changes look good overall, but I've found a few issues. There's a critical bug in the subscription creation sample where the subscription name isn't being set, which will cause it to fail. I also found a potential panic in one of the tests due to incorrect error handling. Additionally, I've included suggestions to improve test robustness and code consistency.