Consolidate fee_amount and amount_needed

Before this commit `fee_amount` and `amount_needed` were passed as independent
parameters. From the perspective of coin selection algorithms, they are always
used jointly for the same purpose, to create a coin selection with a total
effective value greater than it's summed values.

This commit removes the abstraction that the use of the two parameter
introduced by consolidating both into a single parameter, `target_amount`, who
carries their values added up.

Author: csralvall

CHANGELOG.md

@@ -9,6 +9,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Add `Excess` enum to handle remaining amount after coin selection.
 - Move change creation from `Wallet::create_tx` to `CoinSelectionAlgorithm::coin_select`.
 - Change the interface of `SqliteDatabase::new` to accept any type that implement AsRef<Path>
+- Consolidate params `fee_amount` and `amount_needed` in `target_amount` in `CoinSelectionAlgorithm::coin_select` signature.
+- Change the meaning of the `fee_amount` field inside `CoinSelectionResult`: from now on the `fee_amount` will represent only the fees asociated with the utxos in the `selected` field of `CoinSelectionResult`.
 
 ## [v0.20.0] - [v0.19.0]