SPIGOT-3238: Add Server.advancementIterator

By: md_5 <git@md-5.net>

paper-api/src/main/java/org/bukkit/Server.java

@@ -936,6 +936,14 @@ public interface Server extends PluginMessageRecipient {
      */
     Advancement getAdvancement(NamespacedKey key);
 
+    /**
+     * Get an iterator through all advancements. Advancements cannot be removed
+     * from this iterator,
+     *
+     * @return an advancement iterator
+     */
+    Iterator<Advancement> advancementIterator();
+
     /**
      * @see UnsafeValues
      * @return the unsafe values instance