Add models and sdk services example

test/sample_sdk_test.dart

@@ -0,0 +1,33 @@
+import 'package:sample_sdk/sdk.dart';
+import 'package:sample_sdk/sdk/models/category.dart';
+import 'package:sample_sdk/sdk/models/product.dart';
+
+void main() async {
+  final sdk = ClientSDK();
+
+  final category = Category(
+    id: 4,
+    name: 'Category 4',
+    parent: Category(
+      id: 1,
+      name: 'Category 1',
+    ),
+  );
+
+  await sdk.categoryService.create(category);
+  final categories = await sdk.categoryService.findAll();
+  print(categories);
+
+  final product = Product(
+    id: 4,
+    name: 'Product 4',
+    price: 100.0,
+    category: category,
+  );
+
+  await sdk.productService.create(product);
+  final products = await sdk.productService.findAll();
+  print(products);
+
+  sdk.dispose();
+}