added message as optional field

geoportal-common/src/main/java/org/gcube/application/geoportal/common/model/rest/StepExecutionRequest.java

@@ -1,5 +1,6 @@
 package org.gcube.application.geoportal.common.model.rest;
 
+import javax.xml.bind.annotation.XmlAttribute;
 import javax.xml.bind.annotation.XmlRootElement;
 
 import org.bson.Document;
@@ -12,9 +13,10 @@ import lombok.NoArgsConstructor;
 @Data
 @AllArgsConstructor
 @NoArgsConstructor
-public class StepExecutionRequest implements StepExecutionInterface {
+public class StepExecutionRequest {
 
     private String stepID;
+    @XmlAttribute(required = false)
     private String message;
     private Document options;
 }