sysutils/nut: Only one remote monitor can be used

[yodaldevoid]

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• I have read the contributing guide lines at https://github.com/opnsense/plugins/blob/master/CONTRIBUTING.md
• I have searched the existing issues, open and closed, and I'm convinced that mine is new.
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Is your feature request related to a problem? Please describe.
If an OPNsense box depends on more than one UPS managed by a remote system, it cannot monitor all at once when using the plugin.

Describe the solution you'd like
The existing netclient "UPS" should be duplicated to support defining any number of netclients/monitors. This could be displayed in a list, like what is done for firewall rules.

Describe alternatives you've considered
The plugin could be ignored completely and NUT configured manually.