By default, it's assumed that the IPs allocated from the pool are in the
same subnet as the Node IPs. In some cases, users want to use IPs in
different subnets as Egress IPs. Additionally, users may want to use
VLAN taggaing to segment the Egress traffic and the Node traffic.

The commit implements the requirements by introducing an optional field,
`subnetInfo`, to the ExternalIPPool resource. The `subnetInfo` field
contains the subnet attributes of the IPs in this pool. When using a
different subnet:

* `gateway` and `prefixLength` must be set. Antrea will route Egress
  traffic to the specified gateway when the destination is not in the
  same subnet of the Egress IP, otherwise route it to the destination
  directly.

* Optionally, you can specify `vlan` if the underlying network is
  expecting it. Once set, Antrea will tag Egress traffic leaving the
  Egress Node with the specified VLAN ID. Correspondingly, it's
  expected that reply traffic towards these Egress IPs are also tagged
  with the specified VLAN ID when arriving the Egress Node.

The implementation involves VLAN sub-interfaces and policy routing.

* For a given subnet with a VLAN ID, a separate VLAN sub-interface will
  be created to hold the Egress IPs allocated from it. Egress traffic
  and its reply traffic will be sent over and received from the VLAN
  sub-interface for proper tagging and untagging.

* For a given subnet, a separate route table will be created, routing
  the selected Egress traffic to the specified gateway, or to its
  neighbor.

* For multiple Egress IPs associated allocated from the same subnet, a
  separate IP rule will be created for each Egress IP, matching its pkt
  mark and looking up the shared route table.

The feature is gated by the alpha "EgressSeparateSubnet" feature gate.

Signed-off-by: Quan Tian <[email protected]>

Signed-off-by: Quan Tian <[email protected]>