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/interface-definitions/include/firewall/global-options.xml.i b/interface-definitions/include/firewall/global-options.xml.i
index e655cd6ac..03c07e657 100644
--- a/interface-definitions/include/firewall/global-options.xml.i
+++ b/interface-definitions/include/firewall/global-options.xml.i
@@ -271,6 +271,7 @@
       </properties>
       <defaultValue>disable</defaultValue>
     </leafNode>
+    #include <include/firewall/flow-offload.xml.i>
   </children>
 </node>
 <!-- include end -->