security/tailscale: Set auth key to optional in UI

[sheridans]

Allowing the pre-authentication key as an optional field, will allow tailscale registration using either a pre-auth of key or the AuthURL obtained from the status page if enabled without the auth key.

Quality of life improvement to remove the need to create pre-auth keys manually first, users will still need to set the machine key to not expire after regristration in the tailscale dashboard