[FR] - Ability to set auth domains via admin SDK

**Is your feature request related to a problem? Please describe.**
We're trying to deploy preview branches onto appengine using versioning. This results in URLs like `http://[VERSION_ID]-dot-[YOUR_PROJECT_ID].appspot.com`. We want to be able to run E2E tests against preview branches and send links to stakeholders for previewing. Unfortunately with Firebase authentication, we have to manually set each whitelisted subdomain we want to use. This isn't the end of the world but it'd be nice to say "hey, we own this appengine (or whatever) namespace, whitelist everything under it."

**Describe the solution you'd like**
I'd like to either:
1. In Firebase control panel, be able to wildcard a path for authentication. eg. `*-dot-[YOUR_PROJECT_ID].appspot.com`
1. Using Firebase-admin SDK, we'd want to programmatically set a whitelisted domain for authentication. Something along the lines of `admin.auth().validateDomain(...)` might work for our use case.

**Describe alternatives you've considered**
As an alternative, we've:
- thought about trying to proxy our GAE dev instance via some CDN black magic where domains actually all become fourth level domains (unclear if this would even work)
- thought about manually adding whitelisted domains in Firebase for every PR we push 😭

**Additional context**
Just to be crystal clear, this is for the domain whitelisting that happens at https://console.firebase.google.com/u/0/project/[PROJECT_ID]/authentication/providers