Allow gpctl to authenticate from a service account token

[mrsimonemms]

Currently, gpctl can only authenticate via a kubeconfig file. In order to be able to be bundled in the Installer to stop workspaces/image builds prior to a deployment, it must be amended to enable authentication via a service account token.

As an example of how this could work, see the authClusterOrKubeconfig function in the Installer. This tries to authenticate via a service account token and, if that errors, it uses the kubeconfig.

[mrsimonemms]

Found a workaround