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.

src/wallet/mod.rs

@@ -802,11 +802,10 @@ where
             required_utxos,
             optional_utxos,
             fee_rate,
-            outgoing,
-            fee_amount,
+            outgoing + fee_amount,
             &drain_script,
         )?;
-        let mut fee_amount = coin_selection.fee_amount;
+        fee_amount += coin_selection.fee_amount;
         let excess = &coin_selection.excess;
 
         tx.input = coin_selection