Add up ctp connect and up ctp disconnect subcommands for cloud and spaces

[tnthornton]

What problem are you facing?

Now that there has been some focus on bringing CLI functionality from Cloud on par with Spaces, a noticeable missing feature in both places in a nice, succinct, command that would allow an operator to easily "connect" or "disconnect" to/from a target control plane.

How could Upbound help solve your problem?

At a very high level this could be described as:

• up ctp connect ctp-one - this would acquire the kubeconfig for ctp-one, add it to the current kubeconfig and change the context to point to that control plane. That way any subsequent kubectl commands would be invoked against the control plane.
• up ctp disconnect - this would change the kube context back to the original context and any subsequent kubectl commands would be invoked against the original cluster.