Use Cobra flag helpers for better shell completion

[imjasonh]

Cobra has a helper methods which can make shell completion a little nicer:

• MarkFlagFilename tells Cobra that the flag represents a filename. This tells the ko completion logic to generate helpers to match filenames. (e.g., ko apply -f <tab>)
• MarkFlagRequired tells Cobra that the flag is required, and prioritizes in completion if it's unset (e.g., ko apply <tab> prioritizes -f over --preserve-import-paths).

Audit flags we configure and see if there are any that would benefit from being marked this way. Check that the expected behavior is actually observed when shell completion is configured.

[github-actions]

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[imjasonh]

I still want this.

[MohabMohamed]

Hello @imjasonh , is this issue still available?
I would love to work on it

[imjasonh]

Hello @imjasonh , is this issue still available? I would love to work on it

Go for it! Let me know if you have any questions, I'd be happy to help.

And thanks!

[MohabMohamed]

Thank you

[MohabMohamed]

@imjasonh I made the required change in PR #705, but are there any flags other than -f that need to be required?
as I didn't use ko before.