Idea: LSPS to submit failed payments

[ZmnSCPxj-jr]

A custodial wallet has the advantage that they can see every payment of their clients, and from there, get data on which destinations are popular. The custodial wallet can then take that data to drive decision on where to put their channels.

An LSP that supports non-custodial wallets is at a disadvantage in that if a payment fails, the LSP does not know how to fix the issue because it does not know who the ultimate destination is.

Thus I propose to add an advisory interface where the client can say "I tried to reach X, here is the invoice that failed". The LSP can then double-check that (do not trust, verify) by probing to the destination, as well as checking its logs if the client did indeed ask to forward a payment with a matching hash (?), and if indeed the LSP cannot reach the destination for the amount, that is a point of data to let the LSP drive its decisions of where to open their channels.

Care must be taken to prevent this from being abused by fake clients trying to drive liquidity towards their real nodes for free, though I should note that also is possible with custodial wallets (put some funds in custodial wallet, keep sending to your own node until all your channels saturate, and since it is your own node you do not lose the funds, wait for custodial walelt to notice the large number of failed payments to your node and decide to build a channel there).

Proposed API name lspsX.lodge_complaint