Add Delete/List command for Stage

[devholic]

This PR adds Delete/List command for Stage to CLI.

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[codecov]

Codecov Report

Merging #484 (5159f1f) into main (38bc48d) will not change coverage.
The diff coverage is n/a.

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  Lines        4271     4271           
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  Hits         2114     2114           
  Misses       2070     2070           
  Partials       87       87           

[krancour]

Implementation looks fine, but I have a concern about the UX...

In #489, for PromotionPolicies, I see a get command that, as with kubectl, pulls double-duty as list. If a name is specified, it's a get and if a name is not specified, it's a list.

Here I see list added and get is conspicuously absent.

I think we need to pick one style and apply it consistently:

Either get and list are always separate commands OR get is implicitly a list command when no name is specified.

[devholic]

@krancour I created an issue for this: #488

I'll merge list command to get to work same as kubectl in another PR.

[krancour]

LGTM assuming #488 will address the minor concerns that were raised.