feat(pubsub): rewrite v2 schema samples

[hongalex]

This is part 3 of 3 PRs that will be updating our existing Go Pub/Sub samples to use the new v2 library. This PR focuses on updating schema samples and the tests associated with them.

We are merging these PRs into pubsub-v2-samples-trunk since the v2 library isn't fully released yet.

[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 is the third part of a series aimed at updating existing Go Pub/Sub samples to utilize the new v2 client library. Specifically, it focuses on rewriting schema-related samples and their associated tests to adopt the protobuf-based API of the cloud.google.com/go/pubsub/v2 package, ensuring the samples reflect the latest recommended usage.

Highlights

• Go Pub/Sub v2 Client Migration: The core change involves migrating all Go Pub/Sub schema and related topic samples from the older cloud.google.com/go/pubsub client library to the new cloud.google.com/go/pubsub/v2 client, which utilizes a protobuf-based API.
• Schema API Updates: All schema management operations (create, commit, get, list, delete, rollback revisions) have been updated to use the new pubsub/v2/apiv1.SchemaClient methods, which now accept protobuf request objects (e.g., pubsubpb.CreateSchemaRequest) instead of direct parameters or pubsub.SchemaConfig structs.
• Topic Administration API Updates: Topic creation, deletion, listing, IAM policy management, and updates (including ingestion settings) now leverage the client.TopicAdminClient within the v2 Pub/Sub client, using protobuf request and response structures (e.g., pubsubpb.CreateTopicRequest, pubsubpb.UpdateTopicRequest with FieldMask).
• Publisher and Subscriber Client Refinements: Message publishing operations now use client.Publisher(topicName) for creating publisher clients, and subscription operations use client.Subscriber(subID) for subscriber clients, reflecting changes in the v2 client's API surface for these functionalities.
• Test Suite Alignment: The integration tests for both schema and topic samples have been thoroughly updated to align with the new v2 client API, ensuring compatibility and correctness of the migrated code examples.

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[gemini-code-assist]

Code Review

This pull request migrates the Pub/Sub schema samples to the v2 Go library. I've provided suggestions to fix a bug in schema revision fetching, remove redundant code, and improve test clarity.