Implement connectioncontextkernel/ethernetcontext/macaddress

[edwarnicke]

The Connection.ConnnectionContext.EthernetContext allows the NSE to specify the Source and Dest Mac address.

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 Source and Dest Mac addresses.

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

Helpful notes for implementation:

1. netlink.LinkSetHardwareAddr can be used to set the Hardware address on the interface.
2. Make sure to use a netlink.Handle for the NetNS of the Client/NSE. This can be created using mechutils.ToNetlinkHandle. Example of use
3. The netlink.Link on which you need to set the mac address can be retrieved using:

   handle.LinkByName(mechutils.ToInterfaceName(conn, isClient))
   

   Example of use
4. Once implemented, please add to the connectioncontextkernel/client.go and connectioncontextkernel/server.go.

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