Add support for handling network adapters

[norrland]

Add CRUD for glesys networkadapter.

Create - Create new adapter for a server
Details - Read details about a networkadapter
Edit - Update parameters about a networkadapter
Destroy - Delete a networkadapter

Example:

client := glesys.NewClient("CL12345", "MYTOKEN", "MYAPP/1.0")
adapter, err := client.NetworkAdapters.Create(context.Background(),
        glesys.CreateNetworkAdapterParams{Bandwidth: 100, ServerID:
        "vz12345"})