fix(large-messages): wrap getAllowedBuckets() return in unmodifiableSet

Avoid exposing the internal set reference (SpotBugs EI_EXPOSE_REP).

Author: phipag

powertools-large-messages/src/main/java/software/amazon/lambda/powertools/largemessages/LargeMessageConfig.java

@@ -101,7 +101,7 @@ public void withAllowedBuckets(Set<String> allowedBuckets) {
      * @return the (unmodifiable) set of allowed bucket names, never null
      */
     public Set<String> getAllowedBuckets() {
-        return allowedBuckets;
+        return Collections.unmodifiableSet(allowedBuckets);
     }
 
     // For tests purpose