wallet: change FillPSBT to take sighash as optional

Instead of having the caller have to figure out the correct sane default
to provide to FillPSBT, have FillPSBT do that by having it take the
sighash type as an optional. This further allows it to distinguish
between an explicit sighash type being provided and expecting the
default value to be used.

src/wallet/interfaces.cpp

@@ -383,7 +383,7 @@ public:
         }
         return {};
     }
-    std::optional<PSBTError> fillPSBT(int sighash_type,
+    std::optional<PSBTError> fillPSBT(std::optional<int> sighash_type,
         bool sign,
         bool bip32derivs,
         size_t* n_signed,