Plugin for xymon-client

[kumy]

Important notices
Before you add a new report, we ask you kindly to acknowledge the following:

• 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.
• When the request is meant for an existing plugin, I've added its name to the title.

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

[kumy]

@AdSchellevis xlink to PR #4260