storage: add support for Autoclass

[cojenco]

Autoclass feature at the bucket level, allows for the automatic selection of the best storage class based on object metadata and access patterns. Currently, Autoclass supports new buckets only - a bucket can be enabled for Autoclass only at creation, once disabled it cannot be enabled again

System tests are only required in languages with code changes other than documentation:

• Autoclass can be enabled via Bucket Create
• Autoclass can be disabled via Bucket Patch and Update operations
• Bucket Get operations should return the correct state of Autoclass on the bucket which includes the enabled boolean field and toggleTime timestamp field

Samples demonstrating getting and setting Autoclass feature for a bucket:

• 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.