feat: auth multi tenancy support

[JRaams]

Potential starting point for multi tenancy support for firebase/auth, see #1515

Things to keep in mind:

• Tenant is parsed from the idToken / session cookie token first to decide if getAuth or authForTenant should be used. Afterwards the token gets parsed AGAIN in the validation proces, this time including signature validation. I don't think the impact would be too big since it's just parsing some base64url, but perhaps someone has a better idea for this.
• I added e2e tests for testing signins in the packages/nuxt folder, but this means playwright install has to be executed before those tests will pass. Would this be an issue?
• For server session cookie validation to work, environment variable 'GOOGLE_CLOUD_PROJECT' has to be set or the following error would appear: Unknown Error verifying session cookie Must initialize app with a cert credential or set your Firebase project ID as the GOOGLE_CLOUD_PROJECT environment variable to call verifySessionCookie().
  Fixed this for now by prepending 'cross-env' to the package json scripts, as it seemed inappropriate to use the gitignored .env file to let nuxt handle this