Add option to verify target cluster when running nyl template --apply

[NiklasRosenstein]

When using Nyl with nyl-profile.yaml, you might have NYL_PROFILE set to have it use a profile other than the "default" profile. Without the profiles configuration, the regular kubectl configuration is used (e.g. what you have in ~/.kube/config or KUBECONFIG).

It seems likely that eventually someone will make a mistake and think they're using a different NYL_PROFILE or default Kubernetes context and accidentally run nyl template --apply targeting the wrong Kubernetes cluster.

Hence, I think we should add an option to nyl-project.yaml that defines some kind of allow list of which Kubernetes clusters may be targeted. If the target cluster is not in this list, it should maybe error or at least ask for confirmation.

Notional example:

# nyl-project.yaml
allowed_target_clusters:
- aws-infrastructure-prod

This could be the name of a profile or Kubectl context.