Add support for forward proxy auth. Tested with authentik.

- adds new method (`getUserFromProxyAuth) to server/Auth.js to check for
  relevant environment variables and pull the user from the request headers..
 - adds `/forwardauth` route to call new proxy auth method
 - modified /client/login.vue to call `/forwardauth` and authenticate
  the user if returned by forwardauth
 - add a helper method in `server/utils/parseBool.js` to help check for
  boolean values from environment
 - update the readme.md to explain how enable forward auth.

server/Server.js

@@ -242,6 +242,7 @@ class Server {
     dyanimicRoutes.forEach((route) => router.get(route, (req, res) => res.sendFile(Path.join(distPath, 'index.html'))))
 
     router.post('/login', this.getLoginRateLimiter(), (req, res) => this.auth.login(req, res, this.rssFeedManager.feedsArray))
+    router.post('/forwardauth', (req, res) => this.auth.getUserFromProxyAuth(req, res, this.rssFeedManager.feedsArray))
     router.post('/logout', this.authMiddleware.bind(this), this.logout.bind(this))
     router.post('/init', (req, res) => {
       if (this.db.hasRootUser) {