SPIGOT-6078: Add SmithItemEvent and expand SmithingInventory API

By: Julian van den Berkmortel <julianvdberkmortel@outlook.com>

paper-api/src/main/java/org/bukkit/inventory/SmithingInventory.java

@@ -1,6 +1,33 @@
 package org.bukkit.inventory;
 
+import org.jetbrains.annotations.Nullable;
+
 /**
  * Interface to the inventory of a Smithing table.
  */
-public interface SmithingInventory extends Inventory { }
+public interface SmithingInventory extends Inventory {
+
+    /**
+     * Check what item is in the result slot of this smithing table.
+     *
+     * @return the result item
+     */
+    @Nullable
+    ItemStack getResult();
+
+    /**
+     * Set the item in the result slot of the smithing table
+     *
+     * @param newResult the new result item
+     */
+    void setResult(@Nullable ItemStack newResult);
+
+    /**
+     * Get the current recipe formed on the smithing table, if any.
+     *
+     * @return the recipe, or null if the current contents don't match any
+     * recipe
+     */
+    @Nullable
+    Recipe getRecipe();
+}