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.

src/reference/asciidoc/mqtt.adoc

@@ -403,3 +403,14 @@ Certain application events are published by the adapters.
 * `MqttSubscribedEvent` - published by the inbound adapter after subscribing to the topics.
 
 These events can be received by an `ApplicationListener<MqttIntegrationEvent>` or with an `@EventListener` method.
+
+To determine the source of an event, use the following; you can check the bean name and/or the connect options (to access the server URIs etc).
+
+====
+[source, java]
+----
+MqttPahoComponent source = event.getSourceAsType();
+String beanName = source.getBeanName();
+MqttConnectOptions options = source.getConnectionInfo();
+----
+====