Add support for 802.1ad (QinQ tunneling)

Add support for 802.1ad (QinQ tunneling)

Flow key handling changes:
 - Add VLAN header array in struct flow, to record multiple 802.1q VLAN
   headers.
 - Add dpif multi-VLAN capability probing. If datapath supports
   multi-VLAN, increase the maximum depth of nested OVS_KEY_ATTR_ENCAP.

Refactor VLAN handling in dpif-xlate:
 - Introduce 'xvlan' to track VLAN stack during flow processing.
 - Input and output VLAN translation according to the xbundle type.

Push VLAN action support:
 - Allow ethertype 0x88a8 in VLAN headers and push_vlan action.
 - Support push_vlan on dot1q packets.

Use other_config:vlan-limit in table Open_vSwitch to limit maximum VLANs
that can be matched. This allows us to preserve backwards compatibility.

Add test cases for VLAN depth limit, Multi-VLAN actions and QinQ VLAN
handling

Co-authored-by: Thomas F Herbert <thomasfherbert@gmail.com>
Signed-off-by: Thomas F Herbert <thomasfherbert@gmail.com>
Co-authored-by: Xiao Liang <shaw.leon@gmail.com>
Signed-off-by: Xiao Liang <shaw.leon@gmail.com>
Signed-off-by: Eric Garver <e@erig.me>
Signed-off-by: Ben Pfaff <blp@ovn.org>