License assign sub-command

[jakolehm]

Should exchange a valid license key to a subscription token and store it to pharos-license secret in kube-system namespace.

This replaces license-manager addon functionality.

[kke]

How to update the secret?

• A: use -c cluster.yml + ssh to retrieve kubeconfig and use it to configure k8s-client
• B: use -c cluster.yml + ssh + remote kubectl
• C: parse ENV['KUBECONFIG'] using Pharos::Kube::Config and use the resulting config and the current context in it to configure k8s-client

I have a working implementation of option A currently but as it's already using ssh and #668 will also rely on running the remote kubectl, maybe that should be used here too.

[kke]

Switched implementation to option B, it's also a lot simpler.