Added custom item entity support via mixin.

src/main/java/appeng/hooks/AECustomEntityItem.java

@@ -0,0 +1,16 @@
+package appeng.hooks;
+
+import net.minecraft.entity.Entity;
+import net.minecraft.entity.ItemEntity;
+import net.minecraft.item.ItemStack;
+import net.minecraft.server.world.ServerWorld;
+
+public interface AECustomEntityItem {
+
+    /**
+     * @return Either a new entity (not added to the world yet), or the original one
+     *         to keep it.
+     */
+    Entity replaceItemEntity(ServerWorld world, ItemEntity itemEntity, ItemStack itemStack);
+
+}