Feerate should be returned from Transaction construction

When you get TransactionDetails from `tx_builder.finish()` it has the fee for the overall transaction but does not include the feerate of the resulting transaction. There is no simple way for the caller to replicate this information since the transaction does not have witness data at this point. This is annoying because after constructing the psbt the user should know the feerate before signing it.

This can either be done by returning another value from `builder.finish()` or by adding a (perhaps optional) field to `TransactionDetails`.