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.

examples/org.freedesktop.DBus.ObjectManager/examplemanager-planet1-client-glue.h

@@ -47,7 +47,7 @@ public:
 public:
     std::string Name()
     {
-        return proxy_->getProperty("Name").onInterface(INTERFACE_NAME);
+        return proxy_->getProperty("Name").onInterface(INTERFACE_NAME).get<std::string>();
     }
 
 private: