Implement connectioncontextkernel/ethernetcontext/getmac

[edwarnicke]

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

Because the Forwarder is generally the entity creating the interface for kernel mechanism, if the NSE has not already explicitly set the Dest Mac address (which should then be set correctly by #32), the Forwarder needs to get the mac address from the interface its created and set it in the EthernetContext.

Implementation notes:

1. You need only implement the NetworkServiceServer in this case.
2. Make sure to use a netlink.Handle for the NetNS of the 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. netlink.Link.Attrs().HardwareAddr has the net.HardwareAddress for the interface.
5. I would suggest adding a function to types in a mac.go file:

// ToVppMacAddress - converts *net.HardwareAddr to ethernet_types.MacAddress
func ToVppMacAddress(hardwareAddr *net.HardwareAddr) ethernet_types.MacAddress {
  hwAddr := [6]uint8{}
  copy(hwAddr[:], *hardwareAddr)
  return ethernet_types.MacAddress(hwAddr)
}

to convert net.HardwareAddr to the govpp ethernet_types.MacAddress (also suggested in #33)

6. Once implemented, please add to the connectioncontextkernel/client.go and connectioncontextkernel/server.go.

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