Make firebase auth work with appengine staging

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

https://github.com/google/clusterfuzz/blob/8736b31455d52246153b5fada3fd328b5704218c/configs/test/project.yaml#L48

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.