api: Implement server side of long polling.

This commit adds a handler for GET /events which
stores the response object for the client for emitting
events in the future.

backend/eventRecipients.ts

@@ -0,0 +1,18 @@
+// this module is responsible for handling the clients currently connected to the server.
+// It stores the clients in a Map object, where the key is the client's user_id and the value is the client's http response object.
+
+import { Response } from 'express';
+
+const clients = new Map<string, Response>();
+
+export function addClient(userId: string, res: Response) {
+    clients.set(userId, res);
+}
+
+export function removeClient(userId: string) {
+    clients.delete(userId);
+}
+
+export function getClient(userId: string) {
+    return clients.get(userId);
+}

backend/routes/gameRoutes.js

@@ -0,0 +1,10 @@
+import express from 'express';
+import { addClient } from '../eventRecipients';
+const router = express.Router();
+
+router.get('/events', (req, res) => {
+    addClient('user_id', res);
+});
+
+// the post handler should retrieve the game the user is currently in, and update the game state.
+// The request body contains the event data, as described in ARCHITECTURE.md