[K8s] Use external-ip if configured on nginx ingress

[romilbhardwaj]

On some on-prem clusters, ingresses may be exposed through a NodePort service that is backed by a custom external-ip set by the cluster admin. We should return this ip when queries through sky status --endpoints instead of returning localhost.

This PR adds support for reading external-ip if it is set. Also updates docs for clarity on how to use nginx ingress.

# Before:
$ sky status --endpoints test
8888: http://localhost:30008/skypilot/test-2ea4/8888

# After:
# If external-ip is configured and backed by nodeport svc
$ sky status --endpoints test
8888: http://192.156.211.1:30008/skypilot/test-2ea4/8888 

# If external-ip is not configured and backed by nodeport svc
$ sky status --endpoints test
8888: http://localhost:30008/skypilot/test-2ea4/8888 

This is also helpful for skyserve on nginx ingress, since localhost is not accessible from the skyserve controller.

Cherry-picking this change from #3109 for v0.5 release.

Tested (run the relevant ones):

• Code formatting: bash format.sh
• Manual tests - sky launch a ports yaml on a) k3s cluster with this custom nodeport based ingress setup b) GKE cluster with regular loadbalancer based ingress.

[Michaelvll]

Thanks for fixing this @romilbhardwaj! LGTM.