docs: Create Samples for transfer manager

[sydney-munro]

Samples for the upload many operations only. Will do chunked once Parallel Composite Upload is complete.

[snippet-bot]

Here is the summary of changes.

You are about to add 4 region tags.

• samples/snippets/src/main/java/com/example/storage/transfermanager/DownloadBucket.java:19, tag storage_transfer_manager_download_bucket
• samples/snippets/src/main/java/com/example/storage/transfermanager/DownloadMany.java:19, tag storage_transfer_manager_download_many
• samples/snippets/src/main/java/com/example/storage/transfermanager/UploadDirectory.java:19, tag storage_transfer_manager_upload_directory
• samples/snippets/src/main/java/com/example/storage/transfermanager/UploadMany.java:19, tag storage_transfer_manager_upload_many

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[BenWhitehead]

lgtm from java side, probably want to get andrew to review for the general review.

[andrewsg]

LGTM