T4502: firewall: Add software flow offload using flowtable

The following commands will enable nftables flowtable offload on interfaces eth0 eth1:

```
set firewall global-options flow-offload software interface <name>
set firewall global-options flow-offload hardware interface <name>
```

Generated nftables rules:

```
table inet vyos_offload {
    flowtable VYOS_FLOWTABLE_software {
        hook ingress priority filter - 1; devices = { eth0, eth1, eth2, eth3 };
        counter
    }

    chain VYOS_OFFLOAD_software {
        type filter hook forward priority filter - 1; policy accept;
        ct state { established, related } meta l4proto { tcp, udp } flow add @VYOS_FLOWTABLE_software
    }
}
```

Use this option to count packets and bytes for each offloaded flow:
```
set system conntrack flow-accounting
```

To verify a connection is offloaded, run

```
cat /proc/net/nf_conntrack|grep OFFLOAD
```

This PR follows firewalld's implementation: https://github.com/firewalld/firewalld/blob/e748b97787d685d0ca93f58e8d4292e87d3f0da6/src/firewall/core/nftables.py#L590

A good introduction to nftables flowtable: https://thermalcircle.de/doku.php?id=blog:linux:flowtables_1_a_netfilter_nftables_fastpath

1 files changed, 1 insertions, 0 deletions

diff --git a/data/templates/conntrack/sysctl.conf.j2 b/data/templates/conntrack/sysctl.conf.j2
index 075402c04..3d6fc43f2 100644
--- a/data/templates/conntrack/sysctl.conf.j2
+++ b/data/templates/conntrack/sysctl.conf.j2
@@ -24,3 +24,4 @@ net.netfilter.nf_conntrack_tcp_timeout_time_wait = {{ timeout.tcp.time_wait }}
 net.netfilter.nf_conntrack_udp_timeout = {{ timeout.udp.other }}
 net.netfilter.nf_conntrack_udp_timeout_stream = {{ timeout.udp.stream }}
 
+net.netfilter.nf_conntrack_acct = {{ '1' if flow_accounting is vyos_defined else '0' }}