Add resources and recipes for configuring firewall rules.

This is an alternative implementation of https://github.com/sous-chefs/firewall

The accumulator pattern is used, to collect firewall_rule resources across various recipes, which are then combined by the firewall resource into a single configuration file.

The firewall_rule resources are converted to nftables-rules by the library sys_helpers_firewall. The firewall_rule resources are supposed to be largely compatible with those of the firewall cookbook from the sous-chefs.