importer for OPNsense

[tpurschke]

• api exists and can be used to retrieve firewall config: https://docs.opnsense.org/development/api/core/firewall.html
• central management only part of the non-free business edition, therefore we would need to address both central and distributed management: https://docs.opnsense.org/vendor/deciso/opncentral.html?highlight=management