Document (and test) cosign version output

[znewman01]

H/t to @vEpiphyte who observes (on Slack):

2.1.0 changed to have cosign version output print to stdout instead of stderr. The change happened in this commit:
40dbbd8
Which may come from this kubernetes-sigs/release-utils@v0.7.3...v0.7.4
specifically here
kubernetes-sigs/release-utils#76

Thanks for the detective work!

It would be useful for scripts to be able to rely on this information.
Per our versioning policy, for that to happen we need to:

1. Document the expected STDOUT output of cosign version.
2. Add a test that checks the output and that it's sent to the right stream.

(For a bonus, we could add a cosign version --format=json flag that output this information in a machine-friendly way.)

[vEpiphyte]

Thanks filing this!

[LewisThomas]

I'd like to try to tackle this to dip my feet into cosign codebase and Go

[znewman01]

Nice! This should be a good introduction to the codebase. If when something is underdocumented and confusing, reach out here and we'll see if we can help.

[AppleGamer22]

Since this issue has been quiet for a couple of months, I'd be happy to jump into this issue and suggest documentation updates and commits to the version printing subcommand. Would that be appropriate given this issue is already assigned?

[haydentherapper]

Done, assigned @AppleGamer22

[AppleGamer22]

I started a new branch for this issue on my fork, which at the moment contains the documentation update. I'll open the PR once I get the testing code in shape. I'm happy to open it sooner if this is required.