cli-plugin: implement cd semantics

[sttts]

Workspace change is very much like cd in bash. We should have the same ways to go back and forth between root, orgs and workspaces:

$ kubectl config use-context root
$ kubectl kcp workspace use bigcorp
Current personal workspace is "bigcorp".
$ kubectl kcp workspace use myapp
Current personal workspace is "myapp".
$ kubectl kcp workspace use -
Current personal workspace is "bigcorp".
$ kubectl kcp workspace use ..
Current personal workspace is "root".
$ kubectl kcp workspace use -
Current personal workspace is "bigcorp".

Note: - is implemented, .. is not.

Also typing kubectl kcp workspace is tedious. Maybe we want kubectl kcp ws foo and kubectl kcp ws ...

[sttts]

Fixed by #744.

[sttts]

Duplicate of #632.