chore: samples overhaul

[BenWhitehead]

• Centralize environment variables resolution in new Env utility class
• Leverage StorageITRunner and associated fixtures
• Condense PubSubNotification test classes into one class that can all share a topic
• Replace usages of the word file with blob (GCS doesn't have files, and our library always presents Blob as its name for a gcs object)
• Use TemporaryBucket for tests which depend on specific bucket state.
  1. Avoids contention between tests trying to change bucket state
  2. Avoids the "too many bucket modifications" error from gcs due to bucket mutations
  3. Makes it possible, for running tests in parallel in the future
• Update samples to apply try-with-resource to TransferManager and Storage instances
• Avoid using syntax classes in tests (use storage.x(blob) instead of blob.x())
• Add ITCleanupOldBucketsTest

[snippet-bot]

Here is the summary of changes.

You are about to add 4 region tags.

• samples/snippets/src/main/java/com/example/storage/object/AddBlobOwner.java:19, tag storage_add_file_owner
• samples/snippets/src/main/java/com/example/storage/object/PrintBlobAcl.java:19, tag storage_print_file_acl
• samples/snippets/src/main/java/com/example/storage/object/PrintBlobAclForUser.java:19, tag storage_print_file_acl_for_user
• samples/snippets/src/main/java/com/example/storage/object/RemoveBlobOwner.java:19, tag storage_remove_file_owner

You are about to delete 4 region tags.

• samples/snippets/src/main/java/com/example/storage/object/AddFileOwner.java:19, tag storage_add_file_owner
• samples/snippets/src/main/java/com/example/storage/object/PrintFileAcl.java:19, tag storage_print_file_acl
• samples/snippets/src/main/java/com/example/storage/object/PrintFileAclForUser.java:19, tag storage_print_file_acl_for_user
• samples/snippets/src/main/java/com/example/storage/object/RemoveFileOwner.java:19, tag storage_remove_file_owner

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[generated-files-bot]

Warning: This pull request is touching the following templated files:

• samples/install-without-bom/pom.xml
• samples/snapshot/pom.xml
• samples/snippets/pom.xml