Add storage for forwarded payments

[benthecarman]

Routing nodes and LSPs want to track forwarded payments so they can run accounting on fees earned and track profitability across time. We now store these to make it easier to track and allows for future accounting utils in the future.

This shouldn't effect edge user nodes as they should never be forwarding payments.

Implementation is mostly just copied how we currently handle normal payments and adapted for forwarded payments.

[ldk-reviews-bot]

👋 Thanks for assigning @tnull as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[tnull]

Thanks! I do wonder if we should really enable storing all forwards by default, or rather make this optional.

Also more generally I wonder if users really expect us to store all forwarded payments forever, or if we should only keep the last X entries in the store? Also, with general-purpose HTLC interception coming up, maybe storing forwards might even be something we entirely want to leave entirely to the user after all?

What do you think?

[benthecarman]

Addressed comments.

I think this makes sense to include in ldk-node and not just leaving it up to the user. If we are focusing on LSPs this will be an essential feature, especially if we want to add accounting tools down the line.

I think it can make sense to disable this and/or add a function to prune the storage for it. Maybe just an option that tracks totals per channel rather than individual htlcs