Allow NRI plugins to read the allocated IPs to the pod on the PodSandbox
hooks. This is specially useful for networking daemons that depend on the Pod
IPs assigned via CNI by the container runtime (service mesh, network
policies, ... ) to create seamless integrations without having to modify
the existing environment.

Existing network integrations use techniques like:

- CNI chaining (invasive) as the implementation needs to modify the existing
deployment and can cause disruption due to mutations on the
existing configurations.

- Read Pod Status, that is too late in the process, since the pod
lifecycle does not update the pod status until the containers are
running, but there are applications like network policy
implemetnations or service meshes that needs this information before
the pod is completely created.

- Inspect the host namespaces to gather the information, that is very
intrusive and not portable, creating a tighly coupling between.

Add a nri plugin that can be used as reference implementations for this
integration, the nri plugin just log the interesting networking details
of the pod (ips and network namespace) on the different hooks of the pod lifecucle.

Change-Id: Idb9605c719903d0df51c9a1947e0f838870b5c0b
Signed-off-by: Antonio Ojea <[email protected]>

api: add support for reading Pod IPs