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 has already been implemented in the C++ client library and there is a PR in python; reference PR here

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-java/blob/master/google-cloud-clients/google-cloud-storage/src/main/java/com/google/cloud/storage/StorageClass.java (add ARCHIVE constant).
• Consider adding a case with ARCHIVE to one of the unit tests (BucketInfoTest.java or BlobInfoTest.java).

There are also snippets that reference storage classes in google-cloud-examples/src/main/java/com/google/cloud/examples/storage/snippets/StorageSnippets.java but I don't think we need to add references to ARCHIVE here given that there isn't a specific snippet for each possible value.

@kolea2 or @JesseLovelace could you pick this ticket up?

[tritone]

Closed in favor of googleapis/java-storage#4