Fix SING_SINGLETON_GETTER_NOT_SYNCHRONIZED in BulkScanWorkerManager

- Add volatile keyword to instance field
- Implement double-checked locking for thread-safe singleton initialization
- Resolves SpotBugs issue: SING_SINGLETON_GETTER_NOT_SYNCHRONIZED

Author: ic0ns

src/main/java/de/rub/nds/crawler/core/BulkScanWorkerManager.java

@@ -24,11 +24,15 @@
 
 public class BulkScanWorkerManager {
     private static final Logger LOGGER = LogManager.getLogger();
-    private static BulkScanWorkerManager instance;
+    private static volatile BulkScanWorkerManager instance;
 
     public static BulkScanWorkerManager getInstance() {
         if (instance == null) {
-            instance = new BulkScanWorkerManager();
+            synchronized (BulkScanWorkerManager.class) {
+                if (instance == null) {
+                    instance = new BulkScanWorkerManager();
+                }
+            }
         }
         return instance;
     }