docs: oas (#197)

Adds OpenAPI specification of Storefront and Admin APIs.
Updates docs project.

docs-util/helpers/use-server.js

@@ -0,0 +1,41 @@
+const ServerTestUtil = {
+  server_: null,
+  app_: null,
+
+  setApp: function (app) {
+    this.app_ = app;
+  },
+
+  start: async function () {
+    this.server_ = await new Promise((resolve, reject) => {
+      const s = this.app_.listen(PORT, (err) => {
+        if (err) {
+          reject(err);
+        }
+      });
+      resolve(s);
+    });
+  },
+
+  kill: function () {
+    return new Promise((resolve, _) => {
+      if (this.server_) {
+        this.server_.close(() => resolve());
+      }
+      resolve();
+    });
+  },
+};
+
+const instance = ServerTestUtil;
+
+module.exports = {
+  setApp: function (app) {
+    instance.setApp(app);
+    return instance;
+  },
+
+  useServer: function () {
+    return instance;
+  },
+};