MQTT_NS::buffer, provide access to shared_ptr.

[jonesmz]

I have my own shared_ptr based string class, and I want to convert from MQTT_NS::buffer to my own class.

To do this efficiently, I need direct access to the shared ptr.

inline auto buffer::get_shared_ptr() const& -> shared_ptr_array const&
{
    return lifetime_;
}

inline auto buffer::get_shared_ptr() & -> shared_ptr_array &
{
    return lifetime_;
}

inline auto buffer::get_shared_ptr() && -> shared_ptr_array &&
{
    return std::move(lifetime_);
}