SPIGOT-5311: Add API to get/set item associated with throwable projectiles

By: Lars Dormans <lars.dormans@live.nl>

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

@@ -0,0 +1,22 @@
+package org.bukkit.entity;
+
+import org.bukkit.inventory.ItemStack;
+import org.jetbrains.annotations.NotNull;
+
+public interface ThrowableProjectile extends Projectile {
+
+    /**
+     * Gets the ItemStack the thrown projectile will display.
+     *
+     * @return The thrown item display ItemStack
+     */
+    @NotNull
+    ItemStack getItem();
+
+    /**
+     * Sets the display ItemStack for the thrown projectile.
+     *
+     * @param item ItemStack set to be displayed
+     */
+    void setItem(@NotNull ItemStack item);
+}