Access runtime cluster

[petersutter]

What would you like to be added:
It should be possible to access the runtime cluster using gardenctl

Why is this needed:

[petersutter]

proposals for obtaining the runtime cluster kubeconfig:
a) Similar to the <seedname>.login secret for seeds that contain the kubeconfig (usually with oidc configuration), a similar approach could be done for the runtime cluster. There is a secret for the runtime cluster(s), identified by label.
b) Similar to the garden cluster kubeconfig, the user has to obtain the runtime cluster kubeconfig and the user will add it to the gardenctl configuration