extend MQTT integration support

src/lib/ebus/data.h

@@ -209,6 +209,12 @@ class DataField : public AttributedItem {
    */
   virtual bool isSet() const { return false; }
 
+  /**
+   * Return whether this is a @a ValueListDataField.
+   * @return true if this is a @a DataFieldSet.
+   */
+  virtual bool isList() const { return false; }
+
   /**
    * Factory method for creating new instances.
    * @param isWriteMessage whether the field is part of a write message (default false).
@@ -534,6 +540,9 @@ class ValueListDataField : public SingleDataField {
   // @copydoc
   const ValueListDataField* clone() const override;
 
+  // @copydoc
+  bool isList() const override { return true; }
+
   // @copydoc
   result_t derive(const string& name, PartType partType, int divisor,
       const map<unsigned int, string>& values, map<string, string>* attributes,