Provide header for unused GPIO

[t-aus-m]

Hello there,

I really like the idea of this UPS, and I'd like to build it myself someday. However, I think there is one feature that is missing that would make this project even more versatile: As fas as I understood the schematics, there are still unused GPIOs left on the ESP32 that could be used to extend the use of the device even further. Would it be possible to add a header, maybe even accessible from the outside to the device? For example, I'd use this header to be able to safely shut down my NAS in the event that the UPS is nearly empty.

Thanks for that awesome project! Cheers