sysutils/nut: Support any UPS in diagnostics page and widget

[yodaldevoid]

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Is your feature request related to a problem? Please describe.
This diagnostics page and widget only support the netclient monitor. This make them useless if a user only uses directly connected UPSs.

Describe the solution you'd like
The diagnostics page should have a dropdown/combobox to select the UPS to monitor. The widget should have a configuration page to select the UPS to monitor.

Describe alternatives you've considered
The UPS in question could be moved to a separate system and monitored through netclient.