This makes the concept of a `Provider` wholely implemented in the cli
implementation in cmd/virtual-kubelet.
It allows us to slim down the interfaces used in vkubelet (and
vkubelet/api) to what is actually used there rather than a huge
interface that is only there to serve the CLI's needs.
* Add the /runningpods/ api endpoint
This adds an API endpoint from the kubelet (/runningpods/). It is
an endpoint on kubelet which is considered a "debug" endpoint, so
it might be worth exposing through the options, but by default
it is exposed in most k8s configs AFAICT.