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

For this library, this will just require a documentation update since storage class is just passed as a string and there is not any enum or constant. Any comment mentioning other storage classes should be updated to also mention archive.

I found the following places where the docs should be updated:

• https://github.com/googleapis/google-cloud-php/blob/master/Storage/src/Lifecycle.php#L90
• https://github.com/googleapis/google-cloud-php/blob/master/Storage/src/StorageClient.php#L263
• https://github.com/googleapis/google-cloud-php/blob/master/Storage/src/Bucket.php#L822
• https://github.com/googleapis/google-cloud-php/blob/master/Storage/src/Connection/ServiceDefinition/storage-v1.json#L265 (but I'm assuming this is generated?)

Also, it might be good to update one or more of the tests of storage class to use ARCHIVE.