Adding an example for setting multiple fields based on one flag

[chasain]

I built this example while trying to make rules_k8s a bit cleaner for us, it allows me to define something like

clusters(name = "cluster", clusters = [":minikube"])
cluster(
  name = "minikube",
  image_chroot = "localhost:5000",
  ...
)

Then I can call bazel run //some/deployment:object.apply --cluster=minikube and substitute all the values from my cluster into the object.

[gregestren]

Thanks for this PR, @chasain . I'm just seeing this now and will give it a thorough review soon.

[aiuto]

Friendly ping.

[aiuto]

Another ping, a year later.