azure/s3 gateways: Pass ETag during GET call to avoid data corruption (#11024)

Both Azure & S3 gateways call for object information before returning
the stream of the object, however, the object content/length could be
modified meanwhile, which means it can return a corrupted object.

Use ETag to ensure that the object was not modified during the GET call

cmd/object-api-datatypes.go

@@ -168,6 +168,9 @@ type ObjectInfo struct {
 	// Hex encoded unique entity tag of the object.
 	ETag string
 
+	// The ETag stored in the gateway backend
+	InnerETag string
+
 	// Version ID of this object.
 	VersionID string