Add MQTT client wrapper; wire startup publish; support env-config

.env.example

@@ -0,0 +1,13 @@
+# Example env file for lambdaiot-core
+
+# Server
+SERVER_ADDRESS=0.0.0.0
+SERVER_PORT=8080
+JWT_SECRET=secret
+
+# MQTT
+MQTT_BROKER=tcp://localhost:1883
+MQTT_CLIENT_ID=lambdaiot-core
+MQTT_USERNAME=
+MQTT_PASSWORD=
+MQTT_TOPIC=lambda/iot