feat(ctl): Add support for stacklet credentials subcommand (#108)

* Add `--show-credentials` flag

* Add credential output

* Use `--show-credentials` flag

* First batch of suggestions

* Make credentials output standalone command

The command currently fails with an 404 error. It seems like that the k8s
API is unable to find the requested object.

* fix: stacklet credential command

* Improve arg and option handling, improve output

* Various small stacklet command output improvements

* Adjust error handling

* Remove unused flag

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Co-authored-by: Sebastian Bernauer <sebastian.bernauer@stackable.de>

Author: Techassi