feat: Complete export flow rest API endpoint

packages/backend/src/controllers/api/v1/flows/export-flow.js

@@ -0,0 +1,9 @@
+export default async (request, response) => {
+  const flow = await request.currentUser.authorizedFlows
+    .findById(request.params.flowId)
+    .throwIfNotFound();
+
+  const { exportedFlowAsString, slug } = await flow.export();
+
+  response.status(201).attachment(slug).send(exportedFlowAsString);
+};