add mqtt, decouple i2c and sensor

inc/ha_mqtt.h

@@ -0,0 +1,13 @@
+// ha_mqtt.h
+
+#ifndef HA_MQTT_H
+#define HA_MQTT_H
+
+#include <stdint.h>
+
+void ha_mqtt_init(void);
+void ha_mqtt_publish_sensor_data(int32_t temp_x100, int32_t hum_x100);
+void ha_mqtt_check_and_reconnect(void);
+uint8_t ha_mqtt_is_connected(void);
+
+#endif  // HA_MQTT_H