implement a flag to specify custom crossdomain.xml (#19262)

fixes #16909

cmd/globals.go

@@ -171,6 +171,7 @@ type serverCtxt struct {
 	ReadHeaderTimeout   time.Duration
 	MaxIdleConnsPerHost int
 
+	CrossDomainXML string
 	// The layout of disks as interpreted
 	Layout disksLayout
 }