Support CNI-configured network interfaces

[sipsma]

(This code resolving this is already out, retroactively creating to have an issue to reference)

As part of the design for adding CNI support to Firecracker-containerd, it was decided that most of the functionality should be added to the Go SDK so it can benefit both Firecracker-containerd and potentially other consumers. The relevant design doc can be found here: https://github.com/firecracker-microvm/firecracker-containerd/blob/master/docs/networking.md