Add Support for Azure WAL-G Backups (#1537)

This commit adds support for using an Azure storage account as a backup
location.

It uses the existing GCS functionality as a reference for what to do,
and follows the example set by GCS as closely as possible.

The decision to name the cloud provider key "aws_or_gcp" is unfortunate
while adding support for Azure, but I have left it alone to allow for
this changeset to be backwards compatible.

pkg/util/config/config.go

@@ -167,6 +167,7 @@ type Config struct {
 	KubeIAMRole                            string            `name:"kube_iam_role"`
 	WALGSBucket                            string            `name:"wal_gs_bucket"`
 	GCPCredentials                         string            `name:"gcp_credentials"`
+	WALAZStorageAccount                    string            `name:"wal_az_storage_account"`
 	AdditionalSecretMount                  string            `name:"additional_secret_mount"`
 	AdditionalSecretMountPath              string            `name:"additional_secret_mount_path" default:"/meta/credentials"`
 	EnableEBSGp3Migration                  bool              `name:"enable_ebs_gp3_migration" default:"false"`