Implement connectioncontext/ethernetcontext/getmac

[edwarnicke]

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

Often, the NSE will simply allow vpp itself to set a random Mac address for the interface it is creating. In this case, that Mac address needs to be retrieved to set the EthernetContext.DstMac.

Implementation notes:

1. You need only implement the NetworkServiceServer in this case.
2. SwInterfaceGetMacAddress can be used to retrieve the Mac address of the interface given the swIfIndex.
3. You can retrieve the swIfIndex set by the mechanism chain elements using ifindex.Load. [Example of Use]
4. Use types.ToVPPMacAddress to convert net.HardwareAddr to the govpp ethernet_types.MacAddress
5. Once implemented, please add to the connectioncontext/client.go and connectioncontext/server.go.

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