Make firebase auth work with appengine staging

[oliverchang]

Recent firebase mean that currently it's not possible to log in to staging appengine deployments without changing this config:

clusterfuzz/configs/test/project.yaml

Line 48 in 8736b31

auth_domain: login.custom-domain.com

to point to the staging instance URL.

To make this work seamlessly without having to hand edit configs, we should add an additional staging_auth_domain that's used for staging deployments.