SPIGOT-3238: Add Server.advancementIterator

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

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

@@ -1144,6 +1144,16 @@ public final class Bukkit {
         return server.getAdvancement(key);
     }
 
+    /**
+     * Get an iterator through all advancements. Advancements cannot be removed
+     * from this iterator,
+     *
+     * @return an advancement iterator
+     */
+    public static Iterator<Advancement> advancementIterator() {
+        return server.advancementIterator();
+    }
+
     /**
      * @see UnsafeValues
      * @return the unsafe values instance