Add support for TX-Ultimate switch backlight
* **Please explain your feature request in a one or two sentences.**

The TX-Ultimate has a backlight. Expose this property to Homebridge.

* **Is your feature request related to a problem? Please describe.**

No.

* **Any particular eWeLink devices that this relates to?**

Sonoff TX-Ultimate

* **Anything else?**

Let me know if I can provide further diagnostics / output JSON. 

I will sponsor if this feature is added.

