Storage: Add support for Archive storage class

[tritone]

GCS is adding a storage class called "archive" to the existing options for object storage (coldline, standard, etc). Support for this should be added to this client library.

This should be straightforward as ARCHIVE should just mirror existing valid storage classes. I see the following places where changes are required in order to accomplish this:

• https://github.com/googleapis/google-cloud-python/blob/master/storage/google/cloud/storage/constants.py (add new constant)
• https://github.com/googleapis/google-cloud-python/blob/master/storage/google/cloud/storage/bucket.py (add import, update STORAGE_CLASSES, update documentation).
• https://github.com/googleapis/google-cloud-python/blob/master/storage/google/cloud/storage/blob.py (same as above)
• Incorporate ARCHIVE into unit tests as appropriate.

This has already been implemented in the C++ client library; reference commit here

[tseaver]

@tritone What should the descriptive text be for the ARCHIVE_STORAGE_CLASS constant?

[tseaver]

@tritone I made a guess at the descriptive text here:

google-cloud-python/storage/google/cloud/storage/constants.py

Line 28 in d31eeb9

"""Storage class for objects accessed less frequently than once per year."""

[tritone]

Hi @tseaver, yep that is correct! For additional context, I shared you on a doc with some relevant info; also, the preliminary public description is in the blog post here: https://cloud.google.com/blog/products/storage-data-transfer/whats-cooler-than-being-cool-ice-cold-archive-storage .

We may also want to add a link to the public documentation at https://cloud.google.com/storage/docs/storage-classes since there are specific policies around availability and minimum storage duration for each class (archive will be added here once launched).

[tseaver]

@tritone I added the link to all storage class constants in 8272f9f .