ergonomically use local_resource to run a port-forward server

A common question we get is "I have a pod running in Kubernetes that Tilt isn't monitoring the status of...can Tilt create a port-forwarder for this?"

You can do this today! But you have to flip how you frame the question. It's not "how do you tell tilt to connect to this pod". It's "a port-forwarder is a local server. how do i run a local server?"

The solution will look something like this:

local_resource('forwarder', serve_cmd='kubectl port-forward service/my-service 8000:8000')

I feel like there should be a more ergonomic way to figure this out on your own. A Tilt extension might be a good fit

Start with the local_resource entry point and its serve_cmd option, using the kubectl port-forward example in the issue as the behavior to understand. Determine whether a more ergonomic local-resource interface or a Tilt extension best expresses this workflow; done means users can create this port-forward server without needing to reframe the problem themselves.