datapath-windows: Add support for 'OVS_KEY_ATTR_ENCAP' key attribute.

datapath-windows: Add support for 'OVS_KEY_ATTR_ENCAP' key attribute.

Add a new structure in l2 header to accomodate vlan header,
based of commit "d7efce7beff25052bd9083419200e1a47f0d6066
datapath: 802.1AD Flow handling, actions, vlan parsing, netlink attributes"

Also reset vlan header in flow key, after deleting vlan tag from nbl

With this change a sample vlan flow would look like,
eth(src=0a:ea:8a:24:03:86,dst=0a:cd:fa:4d:15:5c),in_port(3),eth_type(0x8100),
vlan(vid=2239,pcp=0),encap(eth_type(0x0800),ipv4(src=13.12.11.149,dst=13.12.11.107,
proto=1,tos=0,ttl=128,frag=no),icmp(type=8,code=0))

Signed-off-by: Anand Kumar <kumaranand@vmware.com>
Acked-by: Alin Gabriel Serdean <aserdean@ovn.org>
Signed-off-by: Alin Gabriel Serdean <aserdean@ovn.org>