Make firebase auth work with appengine staging #4269
Description
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
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.