Allow clustering with Kubernetes

[aleDsz]

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[github-actions]

Uffizzi Preview deployment-53430 was deleted.

[josevalim]

Thanks @aleDsz.

I think we should give up on this idea. We can't make this work by default because it requires defining a service on k8s. If we have to define a service, it may be better to provide users with a Helm chart that starts Livebook, defines the correct env vars and services.

Perhaps our clustered deployment settings should say: "Automatic clustering is available in Fly.io and through our provided Helm charts."

[aleDsz]

I'm going with this k8s idea and creating an example YAML file where it will be possible to create a service, define the environment variables and have the cluster with all the necessary information.