Add mDNS attribute

src/AirGradient.h

@@ -107,6 +107,13 @@ public:
    */
   String getVersion(void);
 
+  /**
+   * @brief Get the Board Name object
+   * 
+   * @return String 
+   */
+  String getBoardName(void);
+
   /**
    * @brief Round double value with for 2 decimal
    *