Method observer

src/rpp/rpp/observers/observer.hpp:179–182

        template<typename... Args>
            requires constraint::is_constructible_from<Strategy, Args&&...>
        explicit observer(DisposableStrategy strategy, Args&&... args)
            : Base{std::move(strategy), std::forward<Args>(args)...}
        {
        }

        template<typename... Args>
            requires (constraint::is_constructible_from<Strategy, Args && ...> && !rpp::constraint::variadic_decayed_same_as<observer, Args...>)