Add keyword mutation for product (#19)

src/resolvers/products-resolver.ts

@@ -94,4 +94,11 @@ export const productMutationTypeDefs = {
     );
     return (<ProductAPI>dataSources.productApi).getProduct(productId);
   },
+  async productKeywords(_, { productId, keywordIds }, { dataSources }) {
+    await (<KeywordAPI>dataSources.keywordApi).setProductKeywords(
+      productId,
+      keywordIds,
+    );
+    return (<ProductAPI>dataSources.productApi).getProduct(productId);
+  },
 };