add heartbeat interval to mqtt settings

interface/src/types/mqtt.ts

@@ -35,6 +35,7 @@ export interface MqttSettings {
   publish_time_mixer: number;
   publish_time_other: number;
   publish_time_sensor: number;
+  publish_time_heartbeat: number;
   mqtt_qos: number;
   mqtt_retain: boolean;
   ha_enabled: boolean;