luci-app-nlbwmon: Provide option to host oui.json locally

[nwf]

I understand why it wasn't included in the source, as it's dynamic and moreover gzips down to 335K, which is a little big to be included in the router firmware. However, given that nlbwmon is likely to be paired with some extended storage for its databases, it'd be nice if the file could be served from some local path rather than going and hitting github every time.

Thanks! --nwf;

[raenye]

Having this file hosted on-prem (i.e., on a LAN-accessible webserver) is also an option.

It would even make sense to have an OpenWrt OUI provider, used by nlbwmon and other packages, with configuration for offline (on-device) storage (partial or full) and online resolution (on-prem and/or web).