Make `currentUser` and `firebaseApp` available in NextJS App Router apps

Per [the docs](https://firebase.google.com/docs/hosting/frameworks/nextjs#optional_integrate_with_firebase_authentication):

> The Express `res.locals` object will optionally contain an authenticated Firebase App instance (`firebaseApp`) and the currently signed-in user (`currentUser`). This can be accessed in `getServerSideProps`.

However, [App Router](https://nextjs.org/docs#app-router-vs-pages-router) apps don't have `getServerSideProps`, so as far as I can tell there isn't a way to access the user on the server in App Router apps. This means that web frameworks' server-side magic that allows the client SDK to be used so that security rules are respected doesn't work.

Not sure how to surface this in the new App Router world, but having a way to access `currentUser` and the `firebaseApp` associated with that user is critical for doing any data access on server render with NextJS