Support VLAN tunnel mapping of VLAN aware bridges
https://legacy.netdevconf.info/2.2/slides/prabhu-linuxbridge-tutorial.pdf
This feature needs to start the external control and then add the FDB database
It is directly controlled by the forwarding database, so it must have the external flag to make the vxlan tunnel work in the metadata collection mode
example:
```
set interfaces vxlan vxlan0 external
set interfaces vxlan vxlan0 source-address <source>
set interfaces vxlan vxlan0 mtu 1500
commit
set interfaces bridge br1 enable-vlan
set interfaces bridge br1 vif 20
set interfaces bridge br1 member interface vxlan0 vlan-tunnel 2000 vlan 20
set interfaces bridge br1 member interface vxlan0 vlan-tunnel 2000 remote <remote>
commit
```