Allow to only send the key to the software-add-on api.
When using the woocommerce-software-addon you can only check a licence key for validity when you send the email address and the product id together with the key.
It would be more convenient to only send the licence key to have a basic check whether this key exists and if it is activated. So people who bought the software only need to enter the key.
All other information is in the backend, so we don't need to send the mail-address and product id every time and the user does not need to enter it.