FR: Auth across subdomains

### [REQUIRED] Describe your environment

* Operating System version: macOS Mojave Version 10.14.6
* Browser version: Google Chrome Version 74.0.3729.131
* Firebase SDK version: 5.5.9
* Firebase Product: auth

### [REQUIRED] Describe the problem

#### Steps to reproduce:
We've transitioned to using Firebase for auth and the desire is to now use Firebase across subdomains, which Firebase does not support out of the box. I've done my research and I see that there's been a few forum items requesting this exact feature [here](https://groups.google.com/forum/#!searchin/firebase-talk/subdomains|sort:date/firebase-talk/LL4p41s4a4I/MD9e3pXZBAAJ) and [here](https://groups.google.com/forum/#!searchin/firebase-talk/subdomains|sort:date/firebase-talk/ljTyEQnpyMo/YbkRILNFAgAJ). Ultimately, the posts link to [this stackoverflow issue](https://stackoverflow.com/questions/45060642/is-there-any-way-to-keep-a-person-authenticated-with-firebase-across-subdomains). As far as I can tell, the best reason for this is:

> Firebase only supports single host origin sessions

Why is this a limitation for Firebase? The stackoverflow item also states that

> Firebase Auth is looking into supporting cookies.

What is the status of this? Has it been funded? Is there a way to see progress of this project somewhere? We have a strong desire to avoid rolling our own subdomain auth system, so looking forward to hearing your response 🙂