add custom facts

[fraenki]

This adds custom facts that look similar to the "os" facts hash, providing basic details about the installed OPNsense release and installed plugins:

$ facter -p
[...]
opnsense => {
  architecture => "amd64",
  name => "OPNsense",
  release => {
    flavour => "OpenSSL",
    full => "21.7.1",
    hash => "ec466867c",
    major => "21.7",
    minor => "1"
  },
  plugins => [
    "os-api-backup",
    "os-dmidecode",
    "os-firewall",
    "os-lldpd",
    "os-haproxy",
    "os-relayd",
    "os-smart",
    "os-wol",
    "os-qemu-guest-agent",
    "os-bind",
    "os-zabbix-agent",
    "os-zabbix5-proxy",
    "os-nginx",
    "os-postfix",
    "os-wireguard"
  ]
}

[codecov]

Codecov Report

Merging #6 (139a02d) into main (3c75dc7) will decrease coverage by 10.29%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##              main       #6       +/-   ##
============================================
- Coverage   100.00%   89.70%   -10.30%     
============================================
  Files           10       11        +1     
  Lines          244      272       +28     
============================================
  Hits           244      244               
- Misses           0       28       +28     

Impacted Files	Coverage Δ
lib/facter/opnsense.rb	0.00% <0.00%> (ø)

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[andreas-stuerz]

Could you please add the missing test so i can merge the PR?
Thx for the contribution!