refactor: interfaces, make 'createTransaction' less error-prone

Bundle all function's outputs inside the util::Result returned object.

Reasons for the refactoring:
- The 'change_pos' ref argument has been a source of bugs in the past.
- The 'fee' ref argument is currently only set when the transaction creation process succeeds.

src/interfaces/wallet.h

@@ -40,6 +40,7 @@ namespace node {
 enum class TransactionError;
 } // namespace node
 namespace wallet {
+struct CreatedTransactionResult;
 class CCoinControl;
 class CWallet;
 enum class AddressPurpose;
@@ -142,11 +143,10 @@ public:
     virtual void listLockedCoins(std::vector<COutPoint>& outputs) = 0;
 
     //! Create transaction.
-    virtual util::Result<CTransactionRef> createTransaction(const std::vector<wallet::CRecipient>& recipients,
+    virtual util::Result<wallet::CreatedTransactionResult> createTransaction(const std::vector<wallet::CRecipient>& recipients,
         const wallet::CCoinControl& coin_control,
         bool sign,
-        int& change_pos,
-        CAmount& fee) = 0;
+        std::optional<unsigned int> change_pos) = 0;
 
     //! Commit transaction.
     virtual void commitTransaction(CTransactionRef tx,