This changes the `/ephemeralcontainers` subresource of `/pods` to use
the `Pod` kind rather than `EphemeralContainers`.
When designing this API initially it seemed preferable to create a new
kind containing only the pod's ephemeral containers, similar to how
binding and scaling work.
It later became clear that this made admission control more difficult
because the controller wouldn't be presented with the entire Pod, so we
updated this to operate on the entire Pod, similar to how `/status`
works.
d22dc5cb72