Provide a way to fetch Pod IP and port in `KubernetesEndpointGroup`

[ikhoon]

KubernetesEndpointGroup was originally introduced to access Kubernetes Pod from outside a Kubernetes cluster. However, there was a request to use KubernetesEndpointGroup for communication within a Kubernetes cluster.

CSLB within a Kubernetes cluster is already possible using DnsAddressEndpointGroup, but DNS propagation can be slow. The requirement is to use KubernetesEndpointGroup to quickly fetch updates via Kubernetes API and shift traffic during rolling restart.