fix: make Variant conversion operator explicit (#428)

Having explicit conversion operator is a good practice according to the C++ core guidelines, as it makes the code safer and better follows the principle of least astonishment. Also, it is consistent with the standard library style, where wrappers like std::variant, std::any, std::optional... also do not provide an implicit conversion to the underlying type. Last but not least, it paves the way for the upcoming std::variant <-> sdbus::Variant implicit conversions without surprising behavior in some edge cases.

include/sdbus-c++/Types.h

@@ -87,7 +87,7 @@ namespace sdbus {
 
         // Only allow conversion operator for true D-Bus type representations in C++
         template <typename _ValueType, typename = std::enable_if_t<signature_of<_ValueType>::is_valid>>
-        operator _ValueType() const
+        explicit operator _ValueType() const
         {
             return get<_ValueType>();
         }