Implement connectioncontextkernel/ipcontext/neighbor

[edwarnicke]

The Connection.ConnectionContext.IPContext.IPNeigbors allows the NSE to specify IPNeighbors associating an IP address to a hardware address that should be configured.

Because manipulating the ethernet address on kernel interfaces requires privilege, and the presumption is that the Forwarder may be the only privileged entity involved in the connection, it falls to the Forwarder in the case of kernel mechanisms to setup those kernel IP Neighbors in the NetNS of the Client/NSE if kernel mechanism is used.

This is in direct analogy to the already existing connectioncontextkernel/ipcontext/ipaddress.

Helpful notes for implementation:

1. netlink.NeighAdd can be used to set create neighbors, and netlink.NeighDel can be used to delete them on Close of the Connection.
2. It is likely a good idea to add a helper to api for extracting net.HardwareAddr from IPNeighbor, similar to func (n *IpNeighbor) GetIP() net.IP
3. Once implemented, please add to the connectioncontextkernel/client.go and connectioncontextkernel/server.go.

Running end to end can be done using cmd-forwarder-vpp.