Enable virtual threads support for PubSub consumers

I've been experimenting with virtual threads on our Spring Boot application by using the property `spring.threads.virtual.enabled=true`.

Our application has a PubSub consumer that uses `spring-cloud-gcp-pubsub-stream-binder`, and I've noticed that it ignores the `spring.threads.virtual.enabled` property and keeps using platform threads to receive incoming messages.

Looking at the configuration it seems that the thread pools for consumers are created in the private method [GcpPubSubAutoConfiguration.registerSubscriberThreadPoolSchedulerBeans](https://github.com/GoogleCloudPlatform/spring-cloud-gcp/blob/main/spring-cloud-gcp-autoconfigure/src/main/java/com/google/cloud/spring/autoconfigure/pubsub/GcpPubSubAutoConfiguration.java#L466) and there's no way to customize the ThreadPool.

Spring's `ThreadPoolTaskScheduler` supports using virtual threads by calling its method `setVirtualThreads(true)`.

The ideal solution would be to enable virtual threads support automatically when `spring.threads.virtual.enabled` is set to `true`. Alternatively, making consumer thread pools customizable by the user would also be an acceptable solution.

Start in spring-cloud-gcp-autoconfigure/src/main/java/com/google/cloud/spring/autoconfigure/pubsub/GcpPubSubAutoConfiguration.java, at registerSubscriberThreadPoolSchedulerBeans. Review how the consumer thread pools are created and how ThreadPoolTaskScheduler supports virtual threads, then trace the spring.threads.virtual.enabled property. Done means PubSub consumers honor that property or expose a supported way to customize their thread pools.

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