GH-3340: IntegrationEvents - add getSourceAsType()

Resolves https://github.com/spring-projects/spring-integration/issues/3340

- add common super-interface for MQTT components
- add `getConnectionInfo()` so users can examine server URIs etc

* Reinstate per-adapter URIs - support multiple

* Restore single URL per adapter.

* Code cleanup for previous commit.

spring-integration-core/src/main/java/org/springframework/integration/events/IntegrationEvent.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2013-2019 the original author or authors.
+ * Copyright 2013-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -48,6 +48,18 @@ public abstract class IntegrationEvent extends ApplicationEvent {
 		return this.cause;
 	}
 
+	/**
+	 * Get the source as a specific type; the receiving variable must be declared with the
+	 * correct type.
+	 * @param <T> the type.
+	 * @return the source.
+	 * @since 5.4
+	 */
+	@SuppressWarnings("unchecked")
+	public <T> T getSourceAsType() {
+		return (T) getSource();
+	}
+
 	@Override
 	public String toString() {
 		return this.getClass().getSimpleName() + " [source=" + this.getSource() +