BLE: Add const overload for byte_array_t subscribt operator.

features/FEATURE_BLE/ble/BLETypes.h

@@ -310,6 +310,13 @@ struct byte_array_t {
         return _value[i];
     }
 
+    /**
+     * Subscript operator to access data content
+     */
+    uint8_t operator[](uint8_t i) const {
+        return _value[i];
+    }
+
     /**
      * Return the pointer to the buffer holding data.
      */