Expose fees paid for outbound payments

[tnull]

For outbound payments, we should expose the fees paid in Event::PaymentSuccessful, and possibly also in PaymentDetails, the latter would go hand-in-hand with the BOLT12 integration as it already will touch the data model there.

[tnull]

Given that we already exposed the paid fees in the PaymentSuccessful event, I'm going back-and-forth whether it's worth complicating the PaymentDirection enum just to include this one field. This would not only be ugly but also would require us to switch it from a 'simple' enum type to an enum interface type in UniFFI bindings, which means we'd need to deal with Arcs etc.

As the first part is completed, and I'm not convinced about the the second part, I'm closing this for now.