dbus_proxy-generated proxy type should provide access to underlying `zbus::Proxy`

Similar to #61 (closed) but for client-side API. This includes:

• A getter. Care should be taken to give it a very unique name to avoid conflicts with the D-Bus methods. zbus_proxy is not a good name for example but it ensure we never have a conflict.

• Deref<Target = zbus::Proxy> implementation for the generated proxy struct.