storage: add samples for Autoclass

[cojenco]

Add samples demonstrating getting and setting Autoclass feature for a bucket. Currently, Autoclass supports new buckets only - a bucket can be enabled for Autoclass only at creation, once disabled it cannot be enabled again

• region tags: storage_get_autoclass and storage_set_autoclass

canonical samples (internal): go/gcs-autoclass-canonical-sample
details and coordination (internal): go/gcs-dpe-autoclass
python-storage example: googleapis/python-storage#791

Please wait for approval from myself before merging/releasing so we can coordinate around release dates.