Simplify Returned Value for User Cost Per Swap

In building out flows on Terminal, we've noticed that it can be complicated and convoluted to grab the end cost to the user for Loop swaps (particularly on the Loop In side of things). We often have to calculate the end swap fee using `CostServer`, chain txes, and invoices. Through discussions internally, we've determined that it makes sense to fix this through a change on the client to just deliver a single value for the cost to the user and not require any calculations beyond that.