FR: Allow a custom retry policy for firebase cloud messaging

Hi there!

It seems as though there is currently no way to supply a different retry policy when initializing an app (it's not one of the options specified [here](https://firebase.google.com/docs/reference/admin/python/firebase_admin#initialize_app), like httpTimeout). This would be useful for customization, as we'd like to modify some of the settings set [here](https://github.com/firebase/firebase-admin-python/blob/527a8245e1ba461817be3f38ac142a0563daeb4d/firebase_admin/_http_client.py#L32).

Thank you!