Implement faster xml_template mode using NetBox inventory

[ypid]

Closes: #40

• A concept how users can make changes to the template and still receive updates to it is not clear yet. For now it is expected that you copy the template into the git repo where your Ansible inventory lives and make changes there.

• Set the device file (fetched) to read only to prevent user edits

  A common why to develop the Jinja2 template is to configure a OPNsense, fetch the config and then diff it (using git). Then you might copy parts over to the template. When you have multiple files (all XML), you might confuse one for the other. Setting all such files that are not the template (which is meant to be edited) to read only helps to notice such mistakes earlier.

• The file copyright style complies with https://reuse.software/