wallet: return CreatedTransactionResult from FundTransaction

Instead of using the output parameters, return CreatedTransactionResult
from FundTransaction in the same way that CreateTransaction does.
Additionally, instead of modifying the original CMutableTransaction, the
result from CreateTransactionInternal is used.

src/wallet/test/fuzz/notifications.cpp

@@ -156,10 +156,9 @@ struct FuzzedWallet {
         coin_control.fOverrideFeeRate = fuzzed_data_provider.ConsumeBool();
         // Add solving data (m_external_provider and SelectExternal)?
 
-        CAmount fee_out;
         int change_position{fuzzed_data_provider.ConsumeIntegralInRange<int>(-1, tx.vout.size() - 1)};
         bilingual_str error;
-        (void)FundTransaction(*wallet, tx, fee_out, change_position, error, /*lockUnspents=*/false, subtract_fee_from_outputs, coin_control);
+        (void)FundTransaction(*wallet, tx, change_position, /*lockUnspents=*/false, subtract_fee_from_outputs, coin_control);
     }
 };