Create LNURL Withdraw Invoices - For refunds

[sixside]

Working on a service that allows spending sats over fiat rails (sats in, fiat out). In some situations, the fiat side might fail after we've already collected the sats from the customer. Obviously the right thing to do in this failure situation is to refund them their sats. Right now we are handling this manually. But it would be nice if we could automate this when we detect a fiat-failure and generate a one-time use LNURL Withdraw invoice automatically for the amount of sats they tried to spend.

AFAICT currently phoenixd only allows withdrawing from an LNURL, but not actually issuing an LNURL https://phoenix.acinq.co/server/api#lnurl-withdraw

We might actually want to segment this to a secondary instance of PhoenixD somehow just to segment funds that can leave the system. That's probably an internal process implementation detail, but I thought I'd mention it here just in case it helps account for a use case. For example maybe we have InboundFunds (payments) & OutboundFunds (refunds). OutboundFunds can receive funds internally from InboundFunds any time we detected a fiat failure for that specific amount (plus a small buffer for network fees). That way, at any given time, the Outbound only has sufficient funds to process refunds ONLY so that and risk of loss is limited to just what's on the Outbound server.