Avoid ListBuckets() call instead rely on simple HTTP GET (#8475)

This is to avoid making calls to backend and requiring
gateways to allow permissions for ListBuckets() operation
just for Liveness checks, we can avoid this and make
our liveness checks to be more performant.

cmd/object-api-datatypes.go

@@ -34,6 +34,8 @@ const (
 	BackendFS
 	// Multi disk BackendErasure (single, distributed) backend.
 	BackendErasure
+	// Gateway backend.
+	BackendGateway
 	// Add your own backend.
 )
 
@@ -49,9 +51,12 @@ type StorageInfo struct {
 
 	// Backend type.
 	Backend struct {
-		// Represents various backend types, currently on FS and Erasure.
+		// Represents various backend types, currently on FS, Erasure and Gateway
 		Type BackendType
 
+		// Following fields are only meaningful if BackendType is Gateway.
+		GatewayOnline bool
+
 		// Following fields are only meaningful if BackendType is Erasure.
 		OnlineDisks      madmin.BackendDisks // Online disks during server startup.
 		OfflineDisks     madmin.BackendDisks // Offline disks during server startup.