Add smart cell definition for KinoFLAME with k8s backend

[mruoss]

Once livebook-dev/kino_flame#3 is merged, let's add a definition for Kubernetes backend with the required packages. I guess there is no way to set the "backend" attribute to "k8s", right?

[github-actions]

Uffizzi Preview deployment-55631 was deleted.

[jonatanklosko]

I guess there is no way to set the "backend" attribute to "k8s", right?

The smart cell itself should guess the default "backend" based on the environment. We could do check which backend module is installed, but since Fly is built-in, that's not reliable. I think the best option is to check env vars, same ones we check for the warning. I see you already do that on the PR, so it should be enough, no?

[mruoss]

Have removed yaml_elixir from the deps.