Make entities immutable and create proper update methods that state by signature what can be updated. (#342)

Signed-off-by: kaizimmerm <kai.zimmermann@bosch-si.com>

hawkbit-dmf-api/src/main/java/org/eclipse/hawkbit/dmf/json/model/ActionUpdateStatus.java

@@ -62,7 +62,7 @@ public class ActionUpdateStatus {
             return Collections.emptyList();
         }
 
-        return Collections.unmodifiableList(message);
+        return message;
     }
 
     public boolean addMessage(final String message) {