allow to enter JWT token

The jitsi nextcloud apps wants you in the settings to enter the JWT_APP_SECRET to generate the JWT token. We should also allow that the full JWT token can be entered and is being appended to the URL see https://github.com/nextcloud/jitsi/issues/26#issuecomment-2466109664

This would allow us to give customers who pay for the fairmeeting pro package, but do not want to authenticate via fairlogin, a jwt token instead. We would give the token a lifetime similar to the duration of the package ordered.