Add ssx-quickstart-express example (#96)

* Add ssx-quickstart-express example

* Add README

* Update response message

Co-authored-by: Sam Gbafa <skgbafa@gmail.com>

* Security improvements

* added env example

* Update yarn lock

---------

Co-authored-by: Sam Gbafa <skgbafa@gmail.com>

examples/ssx-quickstart-express/.env.example

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+SSX_SECRET=
+PORT=

examples/ssx-quickstart-express/.gitignore

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+.env
+node_modules

examples/ssx-quickstart-express/README.md

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+# ssx-quickstart-express
+## Overview
+This is a simple example on how to setup a SSX Server with Express.js. For a step by step guide head to
+https://docs.ssx.id/ssx-server-quickstart.
+
+## Running
+Prepare by creating and editing the secrets in the `.env` file. You can use the `.env.example` as a template.
+```bash
+cp .env.example .env
+```
+
+To run the example, first install the dependencies and then start the server.
+```bash
+yarn && yarn start
+```
+
+The ssx-test-dapp is compatible with this example and is a good interface to test it. Note that some functionalities
+might need extra configuration in order to properly work.

examples/ssx-quickstart-express/package.json

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+{
+  "name": "ssx-quickstart-express",
+  "version": "0.1.0",
+  "main": "index.js",
+  "license": "MIT",
+  "private": "true",
+  "scripts": {
+    "start": "ts-node src/index"
+  },
+  "devDependencies": {
+    "@types/cors": "^2.8.13",
+    "@types/express": "^4.17.17",
+    "@types/node": "^18.14.2",
+    "ts-node": "^10.9.1",
+    "typescript": "^4.9.5"
+  },
+  "dependencies": {
+    "@spruceid/ssx-server": "^1.2.2",
+    "cors": "^2.8.5",
+    "dotenv": "^16.0.3",
+    "express": "^4.18.2"
+  }
+}

examples/ssx-quickstart-express/src/index.ts

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+import express from 'express';
+import dotenv from 'dotenv';
+import { SSXServer, SSXExpressMiddleware } from '@spruceid/ssx-server';
+import cors from 'cors';
+
+dotenv.config();
+const app = express();
+
+const ssx = new SSXServer({
+  signingKey: process.env.SSX_SECRET,
+});
+
+const PORT = process.env.PORT || '4000';
+
+app.use(cors({ credentials: true, origin: true }));
+app.use(express.urlencoded({ extended: true }));
+app.use(express.json());
+app.use(SSXExpressMiddleware(ssx));
+
+app.get('/', (_req, res) => {
+  res.send('SSX Server Online\n');
+});
+
+app.listen(PORT, () => {
+  console.log(`⚡️[server]: Server is running at http://localhost:${PORT}`);
+});