GH-9597: Fix Mqttv5PahoMessageHandler.publish() for concurrency

Fixes: #9597
Issue link: #9597

When input channel for the endpoint with a `Mqttv5PahoMessageHandler` is an `ExecutorChannel` and a lot of concurrent messages are sent,
the `Connect already in progress (32110)` exception can be thrown

* Wrap `this.mqttClient.connect()` in the `Mqttv5PahoMessageHandler.publish()` into a `Lock`

**Auto-cherry-pick to `6.3.x` & `6.2.x`**

Author: artembilan

spring-integration-mqtt/src/main/java/org/springframework/integration/mqtt/outbound/Mqttv5PahoMessageHandler.java

@@ -241,7 +241,15 @@ protected void publish(String topic, Object mqttMessage, Message<?> message) {
 		long completionTimeout = getCompletionTimeout();
 		try {
 			if (!this.mqttClient.isConnected()) {
-				this.mqttClient.connect(this.connectionOptions).waitForCompletion(completionTimeout);
+				this.lock.lock();
+				try {
+					if (!this.mqttClient.isConnected()) {
+						this.mqttClient.connect(this.connectionOptions).waitForCompletion(completionTimeout);
+					}
+				}
+				finally {
+					this.lock.unlock();
+				}
 			}
 			IMqttToken token =
 					this.mqttClient.publish(topic, (MqttMessage) mqttMessage, null, this.mqttPublishActionListener);