Implement connectioncontext/ethernetcontext/macaddress

[edwarnicke]

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

When the Client or NSE are vpp, there need to be chain elements to set the mac address on the interface connected for that connection (no matter its mechanism).

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

Helpful notes for implementation:

1. SwInterfaceSetMacAddress allows you to set the mac address for a vpp interface using its swIfIndex
2. You can retrieve the swIfIndex set by the mechanism chain elements using ifindex.Load. Example of Use.
3. Use types.ToVPPMacAddress to convert net.HardwareAddr to the govpp ethernet_types.MacAddress
4. Once implemented, please add to the connectioncontext/client.go and connectioncontext/server.go.

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