Add shell completions for the CLI

[kastl-ars]

Summary

As a user
I want to be able to auto-complete commands using the tab key
So that it is easier to see which commands and arguments are available.

Context

Most kubernetes-related tools offer shell completions for bash, zsh, fish and powershell. Apparently they are easy to implement (but I am not a go developer).

Would be nice if the k8up command also supported them.

Out of Scope

No response

Further links

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Acceptance Criteria

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Implementation Ideas

No response