feat(storage): add autoclass samples

[cojenco]

Fixes #2707

Requires googleapis/google-cloud-go#6828
Tests pass locally with library feature support

[snippet-bot]

Here is the summary of changes.

You are about to add 2 region tags.

• storage/buckets/get_autoclass.go:17, tag storage_get_autoclass
• storage/buckets/set_autoclass.go:17, tag storage_set_autoclass

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[BrennaEpp]

Thanks Cathy :)

[BrennaEpp]

LGTM