Support for kcp workspace navigation

[embik]

kcp has a unit of isolation called "workspace", which is essentially a virtual Kubernetes API sever on a specific subpath of the URL.

Support for it could do a couple of things that kubectl-workspace, the current official kubectl plugin, can't do yet:

• keep track of the "base url" as metadata.
• allow to list and switch workspaces.
• have better context naming than kubectl-workspace, which uses the context name as a marker for "current" and "previous" contexts.

Some of this I'd expect to upstream into the plugin at some point (if other maintainers agree), but kubeconfig-bikeshed can be a second implementation and/or playground to try things out.