Initialization class as argument to EntitlementAgent (elastic#127815)

Preliminary step for test entitlement initialization, extracted from elastic#127814

Author: ldematte

libs/entitlement/agent/src/main/java/org/elasticsearch/entitlement/agent/EntitlementAgent.java

@@ -25,10 +25,16 @@
  */
 public class EntitlementAgent {
 
+    /**
+     * The agent main method
+     * @param agentArgs arguments passed to the agent.For our agent, this is the class to load and use for Entitlement Initialization.
+     *                  See e.g. {@code EntitlementsBootstrap#loadAgent}
+     * @param inst      The {@link Instrumentation} instance to use for injecting Entitlements checks
+     */
     public static void agentmain(String agentArgs, Instrumentation inst) {
         final Class<?> initClazz;
         try {
-            initClazz = Class.forName("org.elasticsearch.entitlement.initialization.EntitlementInitialization");
+            initClazz = Class.forName(agentArgs);
         } catch (ClassNotFoundException e) {
             throw new AssertionError("entitlement agent does could not find EntitlementInitialization", e);
         }

libs/entitlement/src/main/java/org/elasticsearch/entitlement/bootstrap/EntitlementBootstrap.java

@@ -138,7 +138,7 @@ private static void loadAgent(String agentPath) {
         try {
             VirtualMachine vm = VirtualMachine.attach(Long.toString(ProcessHandle.current().pid()));
             try {
-                vm.loadAgent(agentPath);
+                vm.loadAgent(agentPath, EntitlementInitialization.class.getName());
             } finally {
                 vm.detach();
             }
@@ -154,7 +154,7 @@ private static void exportInitializationToAgent() {
         EntitlementInitialization.class.getModule().addExports(initPkg, unnamedModule);
     }
 
-    private static String findAgentJar() {
+    public static String findAgentJar() {
         String propertyName = "es.entitlement.agentJar";
         String propertyValue = System.getProperty(propertyName);
         if (propertyValue != null) {