Add Merchant API

By: Xor Boole <mcyoung@mit.edu>

paper-api/src/main/java/org/bukkit/entity/HumanEntity.java

@@ -96,6 +96,19 @@ public interface HumanEntity extends LivingEntity, AnimalTamer, Permissible, Inv
      */
     public void openInventory(InventoryView inventory);
 
+    /**
+     * Starts a trade between the player and the villager.
+     *
+     * Note that only one player may trade with a villager at once. You must use
+     * the force parameter for this.
+     *
+     * @param trader The merchant to trade with. Cannot be null.
+     * @param force whether to force the trade even if another player is trading
+     * @return The newly opened inventory view, or null if it could not be
+     * opened.
+     */
+    public InventoryView openMerchant(Villager trader, boolean force);
+
     /**
      * Force-closes the currently open inventory view for this player, if any.
      */