Add a flowtable to speed up forwarding

[majst01]

This is another approach to metal-stack/firewall-controller#157

This will only speed up new firewalls, will not change the metal table on existing firewalls.

• https://thermalcircle.de/doku.php?id=blog:linux:flowtables_1_a_netfilter_nftables_fastpath
• https://firewalld.org/2023/05/nftables-flowtable
• nftables wiki https://wiki.nftables.org/wiki-nftables/index.php/Flowtables
• https://www.kernel.org/doc/html/latest/networking/nf_flowtable.html

TODO:

• measure impact
• check if accounting and NAT still works
• enable accounting e.g. sysctl -w net.netfilter.nf_conntrack_acct=1
• check if accounting of external/internal traffic still works