runelite-api: Add Grand Exchange offer support

This includes a new event: GrandExchangeOfferChanged which is fired
whenever a grand exchange offer is updated, either manually by the
player, or as a result of the offer being fufilled (partially or
otherwise)

runescape-api/src/main/java/net/runelite/rs/api/RSGrandExchangeOffer.java

@@ -24,26 +24,33 @@
  */
 package net.runelite.rs.api;
 
+import net.runelite.api.GrandExchangeOffer;
 import net.runelite.mapping.Import;
 
-public interface RSGrandExchangeOffer
+public interface RSGrandExchangeOffer extends GrandExchangeOffer
 {
 	@Import("quantitySold")
+	@Override
 	int getQuantitySold();
 
 	@Import("itemId")
+	@Override
 	int getItemId();
 
 	@Import("totalQuantity")
+	@Override
 	int getTotalQuantity();
 
 	@Import("price")
+	@Override
 	int getPrice();
 
 	@Import("spent")
+	@Override
 	int getSpent();
 
 	@Import("state")
 	byte getRSState();
 
+
 }