chore: switch tunnel to use new wgtunnel

Once the new wgtunnel is merged and deployed at https://github.com/coder/wgtunnel, switch the tunnel code in this repo to use github.com/coder/wgtunnel/tunnelsdk.

We need to ensure that existing tunnels don't get their URL changed by writing a tunnel version to a file and using that version in the tunnel registration request so we get an old format. New tunnels can use the new shorter URL.

Also make the tunnel URL configurable