support vlan-aware bridges (vlan-filtering)

It would be useful for netplan to support vlan-aware bridge configuration. In this configuration a single bridge can forward tagged frames without the need to create per-vlan bridges and interfaces plugged into it.

An example on why this is needed:

4000 VLANs 1-4000 on 32 ports

1) no vlan filtering

total_interfaces = total_physical_ports + total_physical_ports * num_vlans + total_vlan_bridges = 32 + 32 * 4000 + 4000 = 132032 interfaces

2) vlan filtering:

total_interfaces = total_physical_ports + bridge_device + total_bridge_vlan_interfaces = 32 + 1 + 4000 = 4033 interfaces

NOTE: total_bridge_vlan_interfaces is only needed for "switch virtual interfaces" (SVIs) which allow the host/switch itself to communicate on those VLANs.

ifupdown did not support this while ifupdown2 written by cumulus does:
http://manpages.ubuntu.com/manpages/bionic/man5/ifupdown-addons-interfaces.5.html
bridge-vlan-aware

networkd supports this:
https://www.freedesktop.org/software/systemd/man/systemd.netdev.html#VLANFiltering=
VLANFiltering=
A boolean. This setting controls the IFLA_BR_VLAN_FILTERING option in the kernel. If enabled, the bridge will be started in VLAN-filtering mode. When unset, the kernel's default setting applies.

Examples with systemd-networkd:

https://github.com/Mellanox/mlxsw/wiki/Persistent-Configuration#network-configuration
https://github.com/systemd/systemd/issues/8087#issuecomment-378714138