Add support for HotSwapAgent to devtools

HotSwapAgent is an open source competition to JRebel. This commit adds
"org.hotswap.agent.HotswapAgent" to the list of known Java agent based
class reloaders. This causes same behavior as when JRebel is used: a full
restart is NOT triggered when any class changes, however LiveReload event
is still triggered.

See gh-8683

spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/AgentReloader.java

@@ -36,6 +36,7 @@ public abstract class AgentReloader {
 		Set<String> agentClasses = new LinkedHashSet<String>();
 		agentClasses.add("org.zeroturnaround.javarebel.Integration");
 		agentClasses.add("org.zeroturnaround.javarebel.ReloaderFactory");
+		agentClasses.add("org.hotswap.agent.HotswapAgent"); // relevant for http://hotswapagent.org/ (https://github.com/HotswapProjects/HotswapAgent)
 		AGENT_CLASSES = Collections.unmodifiableSet(agentClasses);
 	}