Add timestamp to `ForwardedPayment`

[benthecarman]

lightningdevkit/ldk-node#772 will add proper storage for forwarded payments to ldk-node. When we migrate to this, we should add a timestamp to our ForwardedPayment return type. It shouldn't be exact so we can't doxx past payments, however the ldk-node PR handles that already.

Found this while working on the thunderhub integration