Add support for S3 storage classes

[jonatas-rp]

Closes #1104

Changes:

• Add --storage-class to barman-cloud-backup, --storage-class-wals to barman-cloud-wal-archive, and aws_storage_class aws_storage_class_wals server config keys, all forwarded to S3 uploads as the StorageClass header.
• Raises ObjectIsArchived exception when files are archived with explicit message that hints users to issue an RestoreObject request
• Adds tests to validate configuration variables are correctly being forwarded
• Included documentation for the new added config keys and warnings related to archived objects