INT-4407: Add merge option to Log4j2LevelAdjuster

JIRA: https://jira.spring.io/browse/INT-4407

To make a `Log4j2LevelAdjuster` more builder friendly, add a `merge`
option to the overloaded `classes()` and `categories()` methods.
This way the existing (or previously configured) `categories` and
`classes` are merged with the provided values and default
`org.springframework.integration` category can be reuse.

Note: this behavior can be reconsidered as default one in the `5.1`

spring-integration-jms/src/test/java/org/springframework/integration/jms/JmsOutboundGatewayTests.java

@@ -75,7 +75,7 @@ public class JmsOutboundGatewayTests extends ActiveMQMultiContextTests {
 	@Rule
 	public Log4j2LevelAdjuster adjuster =
 			Log4j2LevelAdjuster.trace()
-					.categories("org.springframework.integration", "org.springframework.jms", "org.apache");
+					.categories(true, "org.springframework.jms", "org.apache");
 
 	@Test
 	public void testContainerBeanNameWhenNoGatewayBeanName() {