Support FastBoot authentication

[mikkopaderes]

Firebase auth uses JWT tokens for authentication, not cookies. JWT tokens are short live so if we pass that in to the server through cookies and verify it, there's a high chance that it's already expired.

Service workers might work although I haven't tried this solution yet.

Here's the flow of I think FastBoot auth could work:

1. Have a service worker that intercepts fetch and modify to add the result of getIdToken()
2. In FastBoot server, verify the token through verifyIdToken().
3. Once the verification succeeds, we generate a custom token through createCustomToken().
4. We then pass-in the custom token in the request header to Ember FastBoot (maybe in Authorization header?)
5. In Ember FastBoot, we access the token through the header and use it in the ESA authenticator restore() hook. We'll be using signInWithCustomToken() to finally sign in the user.

This addon can support points 1 and 5 although I'm still contemplating if we should have 1 built-in. Points 2, 3, and 4 would have to be done in the FastBoot server which is out of our control.

[chadRoberge]

https://github.com/dockyard/ember-service-worker Is an add on I use with my projects and works well. Might be easier to implement that.