refactor: make createPlainMessage public (#448)

Library clients may need to manually create plain sdbus-c++ message to be able to unit test their code.

include/sdbus-c++/Message.h

@@ -332,6 +332,8 @@ namespace sdbus {
         PlainMessage() = default;
     };
 
+    PlainMessage createPlainMessage();
+
     template <typename ...Elements>
     inline Message& Message::operator<<(const std::variant<Elements...>& value)
     {