mqtt: implement Close (#905)

* mqtt: implement Close

Signed-off-by: Long Dai <long0dai@foxmail.com>

* feedback

Signed-off-by: Long Dai <long0dai@foxmail.com>

Co-authored-by: Yaron Schneider <yaronsc@microsoft.com>

bindings/mqtt/mqtt.go

@@ -311,3 +311,12 @@ func (m *MQTT) createClientOptions(uri *url.URL, clientID string) *mqtt.ClientOp
 
 	return opts
 }
+
+func (m *MQTT) Close() error {
+	if m.consumer != nil {
+		m.consumer.Disconnect(1)
+	}
+	m.producer.Disconnect(1)
+
+	return nil
+}