Add support for MQTT server as a notification target (#4474)

This implementation is similar to AMQP notifications:

* Notifications are published on a single topic as a JSON feed
* Topic is configurable, as is the QoS. Uses the paho.mqtt.golang
  library for the mqtt connection, and supports connections over tcp
  and websockets, with optional secure tls support.
* Additionally the minio server configuration has been bumped up
  so mqtt configuration can be added.
* Configuration migration code is added with tests.

MQTT is an ISO standard M2M/IoT messaging protocol and was
originally designed for applications for limited bandwidth
networks. Today it's use is growing in the IoT space.

cmd/bucket-notification-utils.go

@@ -144,6 +144,9 @@ func isValidQueueID(queueARN string) bool {
 	if isAMQPQueue(sqsARN) { // AMQP eueue.
 		amqpN := serverConfig.Notify.GetAMQPByID(sqsARN.AccountID)
 		return amqpN.Enable && amqpN.URL != ""
+	} else if isMQTTQueue(sqsARN) {
+		mqttN := serverConfig.Notify.GetMQTTByID(sqsARN.AccountID)
+		return mqttN.Enable && mqttN.Broker != ""
 	} else if isNATSQueue(sqsARN) {
 		natsN := serverConfig.Notify.GetNATSByID(sqsARN.AccountID)
 		return natsN.Enable && natsN.Address != ""
@@ -251,6 +254,7 @@ func validateNotificationConfig(nConfig notificationConfig) APIErrorCode {
 // Unmarshals input value of AWS ARN format into minioSqs object.
 // Returned value represents minio sqs types, currently supported are
 // - amqp
+// - mqtt
 // - nats
 // - elasticsearch
 // - redis
@@ -273,6 +277,8 @@ func unmarshalSqsARN(queueARN string) (mSqs arnSQS) {
 	switch sqsType {
 	case queueTypeAMQP:
 		mSqs.Type = queueTypeAMQP
+	case queueTypeMQTT:
+		mSqs.Type = queueTypeMQTT
 	case queueTypeNATS:
 		mSqs.Type = queueTypeNATS
 	case queueTypeElastic: