GH-342 - Allow disabling event externalization.

We now expose spring.modulith.events.externalization.enabled that can be set to false to disable the event externalization completely. Useful in test cases, for example.

spring-modulith-events/spring-modulith-events-api/src/main/java/org/springframework/modulith/events/EventExternalizationConfiguration.java

@@ -77,6 +77,15 @@ public interface EventExternalizationConfiguration {
 		return new Selector();
 	}
 
+	/**
+	 * Disables event externalization by not matching any events at all.
+	 *
+	 * @return will never be {@literal null}.
+	 */
+	public static EventExternalizationConfiguration disabled() {
+		return externalizing().select(__ -> false).build();
+	}
+
 	/**
 	 * A {@link Predicate} to select all events annotated as to be externalized. The currently supported annotations are:
 	 * <ul>