Option to do Client only auth with SSR

[alexandresanchograca]

In my usage I found it lacked the need to support client only Auth with SSR. Currently I only want to do authentication on the client, but still want to fetch firestore data that doesn't require an authenticated user to be able to retrieve this data on the server.

This change should be compatible with all the old versions without any breaking changes.
To only allow authentication on the client during SSR:

    auth: {
      enabled: true,
      clientOnly: true
    }

Before this would require an admin service-account to be included in your project so that the app could fetch the server data, with this check we disable the loading of the plugin-authenticate-user.server, and can start a firebase app on the server without firebaseAuth and retrieve public/accessible data of firestore without needing an authorisation.
When on the client the authorisation will remain enabled and the rest of the data that requires authorisation will be loaded client only.