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

request (internal): go/sat-client-library-request details and coordination (internal): go/gcs-dpe-autoclass python-storage example: https://github.com/googleapis/python-storage/pull/791

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