LNURL Pay error "Success Action URL has different domain than the callback domain"

[danielgranhao]

We've found that some LNURL providers are not strictly following LUD-9. Specifically, they use different domains for the action and callback URLs. The SDK doesn't allow this, as defined in the LNURL spec, but it seems most other wallets do.

In out-of-band discussions, we agreed that the main reason for this limitation is trust. LNURL clients should display the domain name, but later, the successful action would take the client to a different domain.

An idea for a solution is to add a field to UrlSuccessActionData that warns about the mismatch but still allows the payment to be made.