feat(api): add CORS headers (#50120)

* test: allow mocking of env vars

Since utils/env is a module, we can mock it to control env vars in
tests. However, it's not compatible with building the server in
setupFilesAfterEnv, so, instead, we can use a utility function to keep
things DRY.

* fix: update type of fastifyTestInstance

* chore: add comment about sts preload

* chore: rename header plugin

* test: add get util + provide origin on request

* feat: add cors headers

* chore: add TODO

api/jest.utils.ts

@@ -0,0 +1,30 @@
+import request from 'supertest';
+
+import { build } from './src/app';
+
+declare global {
+  // eslint-disable-next-line no-var
+  var fastifyTestInstance: Awaited<ReturnType<typeof build>> | undefined;
+}
+
+export function superGet(endpoint: string): request.Test {
+  return request(fastifyTestInstance?.server)
+    .get(endpoint)
+    .set('Origin', 'https://www.freecodecamp.org');
+}
+
+export function setupServer(): void {
+  let fastify: Awaited<ReturnType<typeof build>> | undefined;
+  beforeAll(async () => {
+    fastify = await build();
+    await fastify.ready();
+
+    global.fastifyTestInstance = fastify;
+  });
+
+  afterAll(async () => {
+    // Due to a prisma bug, this is not enough, we need to --force-exit jest:
+    // https://github.com/prisma/prisma/issues/18146
+    await fastifyTestInstance?.close();
+  });
+}