feat: write REST API endpoint to create step

packages/backend/src/controllers/api/v1/flows/create-step.js

@@ -0,0 +1,14 @@
+import { renderObject } from '../../../../helpers/renderer.js';
+
+export default async (request, response) => {
+  const flow = await request.currentUser.authorizedFlows
+    .clone()
+    .findById(request.params.flowId)
+    .throwIfNotFound();
+
+  const createdActionStep = await flow.createActionStep(
+    request.body.previousStepId
+  );
+
+  renderObject(response, createdActionStep, { status: 201 });
+};