OPNsense/PFsense models: Do not remove <?xml header, and add version information as comment.

[pv2b]

Pre-Request Checklist

• Passes rubocop code analysis (try rubocop --auto-correct)
• Tests added or adapted (try rake test)
• Changes are reflected in the documentation
• User-visible changes appended to CHANGELOG.md

Description

This PR does a two things:

1. There was an issue where the OPNsense and PFsense models would chop off the first line of the config file containing an <?xml... header. This is unneccessary and might cause issues, and was probably left in unintentionally, so I removed this.
2. Added functionality to OPNsense and PFsense models to gather version information and put it into a comment.
3. Refactors the "xmlcomment" function added in step 2 into model.rb, since it's probably useful to other models, and it's really messy to keep it in the opnsense and pfsense models.

[ghost]

Can you add the change to changelog please?

[pv2b]

Can you add the change to changelog please?

Done