feat(storage): sample for autoclass

[vishwarajanand]

Addresses: #1682
(couldn't test in local, since feature not enabled in my gcp proj)

Design doc: go/php-storage-autoclass
PR for python: googleapis/python-storage#791

[snippet-bot]

Here is the summary of changes.

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• storage/src/get_bucket_autoclass.php:26, tag storage_get_autoclass
• storage/src/set_bucket_autoclass.php:26, tag storage_set_autoclass

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[vishwarajanand]

Update: To be merged in the first week of Nov.

[cojenco]

Thanks for working on the samples. Deferring to @saranshdhingra for further review