Improve shell completion for convert-config

[albers]

I would like to improve shell completion of kubelogin.
I started with a command and option that I use frequently: kubelogin convert-config --login.

What I did

I found it hard to spot a place where the code naturally fits.
I decided to put it beside the AddFlags methods so that I can access non-exported variables (supportedLogin)
and do not conflict with existing tests.
The structure is intended to be open for further additions.

Please let me know if this is a good way to go before I add more completions.

How to verify

Import bash completion for kubelogin:

source <(kubelogin completion bash)

Without this PR:
kubelogin convert-config <TAB><TAB> => completion of filenames (wrong)
kubelogin convert-config --login <TAB><TAB> => completion of filenames (wrong)

With this PR:
kubelogin convert-config <TAB><TAB> => no completion
kubelogin convert-config --login <TAB><TAB> => completion of supported logins

[albers]

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[bcho]

LGTM