config: support --context for switching between config profiles

based on kubectl --context so that there's no need to run 

```
$ prime config use user1
$ prime sandbox create ...

$ prime config use user2
$ prime sandbox create ...
```

we can run instead:
```
$ prime sandbox --context user1 create ...
$ prime sandbox --context user2 create ...
```

kinda specific for my usecase for testing with multiple users.