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, 6 insertions, 0 deletions

diff --git a/interface-definitions/system-conntrack.xml.in b/interface-definitions/system-conntrack.xml.in
index 3abf9bbf0..78d19090c 100644
--- a/interface-definitions/system-conntrack.xml.in
+++ b/interface-definitions/system-conntrack.xml.in
@@ -9,6 +9,12 @@
           <priority>218</priority>
         </properties>
         <children>
+          <leafNode name="flow-accounting">
+            <properties>
+              <help>Enable connection tracking flow accounting</help>
+              <valueless/>
+            </properties>
+          </leafNode>
           <leafNode name="expect-table-size">
             <properties>
               <help>Size of connection tracking expect table</help>