add v3.4 entity id flag to MQTT settings

lib/framework/MqttSettingsService.h

@@ -61,6 +61,10 @@ static String generateClientId() {
 #define FACTORY_MQTT_MULTIPLE_INSTANCES false
 #endif
 
+#ifndef FACTORY_MQTT_ENTITY_FULLNAME
+#define FACTORY_MQTT_ENTITY_FULLNAME false
+#endif
+
 class MqttSettings {
   public:
     // host and port - if enabled
@@ -82,6 +86,9 @@ class MqttSettings {
     // multiple instances
     bool multiple_instances;
 
+    // use v3.4 entity fullnames in Discovery
+    bool entity_fullname;
+
     // proddy EMS-ESP specific
     String   base;
     uint16_t publish_time_boiler;