docs: Adding a GCS subscription example

[maitrimangal]

Creating a Cloud Storage Subscription example from Pubsub

[snippet-bot]

Here is the summary of changes.

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• samples/snippets/src/main/java/pubsub/CreateCloudStorageSubscriptionExample.java:19, tag pubsub_create_cloud_storage_subscription

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[lahuang4]

Can you please update unit tests to test the new sample? You can look at https://github.com/googleapis/java-pubsub/pull/1168/files for inspiration.

[maitrimangal]

Can you please update unit tests to test the new sample? You can look at https://github.com/googleapis/java-pubsub/pull/1168/files for inspiration.

Testcases will need to be added in a separate PR, because we need to move over our testing env from java-docs-samples-testing to a new env so that we can add the service account needed for this GCS Bucket to the project. Will create a new Issue to address this.

[maitrimangal]

Created issues: #1792 and #1793 to track the test cases.