Allow users to filter on ICMP (v4) type and code: ``` rule icmp.type xxx icmp.code xxx ACCEPT ``` See [iptables](https://ipset.netfilter.org/iptables-extensions.man.html#lbBC) and [nftables](https://wiki.nftables.org/wiki-nftables/index.php/Quick_reference-nftables_in_10_minutes#Icmp) implementation.
