lnurl-pay: no lud-09 ('successAction') support

[f321x]

LUD-09 specifies how a wallet can show a message / url provided by the payee after the lnurl-pay payment has been made. This is used by offline point of sales terminals (e.g. Opago, LNBits PoS) and vending machines to allow the payer to confirm that a payment was made. The user pays a lnurl invoice generated on the (offline) device, then opens a url contained in the the lud-09 successAction message, sees a unique code on the website and enters the code in the PoS terminal/vending machine to confirm that the payment was made. Right now the UX with Electrum is annoying as the user can pay the lnurl but then cannot confirm to the PoS/vending machine that the payment was made, resulting in the payment being lost.