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Is your feature request related to a problem? Please describe.
Add support for xymon-client

Describe the solution you'd like
The plugin would:

• depend on xymon-client package Add net-mgmt/xymon-client tools#434
• propose a page /ui/xymon which should:
  • allow enabling/disabling
  • having an input text to set the xymon server address
  • provide service buttons: start/stop/status
• shown on dashboard Services widget
• sync config for HA

Describe alternatives you've considered
We were downloading the xymon-client pkg manually and installing it on our systems, then had to manually configure them and enable rc script

Additional context
I already worked on such plugin and will open a PR