SPIGOT-5853: Add DragonBattle#generateEndPortal()

By: Parker Hawke <hawkeboyz2@hotmail.com>

paper-api/src/main/java/org/bukkit/boss/DragonBattle.java

@@ -33,11 +33,20 @@ public interface DragonBattle {
      *
      * This location will be at the center of the base (bottom) of the portal.
      *
-     * @return the end portal location
+     * @return the end portal location or null if not generated
      */
-    @NotNull
+    @Nullable
     public Location getEndPortalLocation();
 
+    /**
+     * Generate the end portal.
+     *
+     * @param withPortals whether or not end portal blocks should be generated
+     *
+     * @return true if generated, false if already present
+     */
+    public boolean generateEndPortal(boolean withPortals);
+
     /**
      * Check whether or not the first dragon has been killed already.
      *