Describe the enhancement

BDK does not store the transaction fee in a separate field. It needs to be calculated from the difference between prev outputs and outputs created.

For transactions that are received from an external wallet, we would not have prev txout data by default. It'll be handy to have an option to get prev txouts for these transactions and have them inserted as floating txouts in the TxGraph.

Use case

Some wallets would want to be able to calculate fees for all transactions that it includes.

Additional context