Add example of protecting a route with subscription

[matthewwong525]

Before i do this i probably need to add a pricing page within the app. And before i do that i probably need to #55

To protect a route:

1. In route_notifier.dart, I need to make the redirect, data function async
2. add the route i want to protect in an if statement
3. add a await ref.read(metadataProvider.future) to retrieve the latest metadata data in the if statement (should be cached so it should be relatively fast)
4. Check the subscription in the if statement to protect the route.