Add endpoint for StockItem model

lib/inventree/stock.dart

@@ -0,0 +1,22 @@
+import 'model.dart';
+
+
+class InvenTreeStockItem extends InvenTreeModel {
+  @override
+  String URL = "stock/";
+
+  InvenTreeStockItem() : super();
+
+  InvenTreeStockItem.fromJson(Map<String, dynamic> json) : super.fromJson(json) {
+    // TODO
+  }
+
+  @override
+  InvenTreeModel createFromJson(Map<String, dynamic> json) {
+    var item = InvenTreeStockItem.fromJson(json);
+
+    // TODO?
+
+    return item;
+  }
+}