feat(cmp): Print stderr output from command even on success #15921

[peschmae]

Always log stderr from command execution in cmp sidecar.

If the command executed succesfully, log everything written to stderr to the sidecard in Info level.
This makes developing and debugging plugins easier.

Fixes #15921

Checklist:

• [x ] Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
• [ x] The title of the PR states what changed and the related issues number (used for the release note).
• [x ] The title of the PR conforms to the Toolchain Guide
• [ x] I've included "Closes [ISSUE #]" or "Fixes [ISSUE #]" in the description to automatically close the associated issue.
• I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them.
• [ x] Does this PR require documentation updates?
• [ x] I've updated documentation as required by this PR.
• Optional. My organization is added to USERS.md.
• [ x] I have signed off all my commits as required by DCO
• I have written unit and/or e2e tests for my change. PRs without these are unlikely to be merged.
• My build is green (troubleshooting builds).
• My new feature complies with the feature status guidelines.
• [x ] I have added a brief description of why this PR is necessary and/or what this PR solves.

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (99a4bf0) 49.67% compared to head (d595cf2) 49.66%.
Report is 5 commits behind head on master.

❗ Current head d595cf2 differs from pull request most recent head 9682aea. Consider uploading reports for the commit 9682aea to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #15973      +/-   ##
==========================================
- Coverage   49.67%   49.66%   -0.01%     
==========================================
  Files         267      267              
  Lines       46383    46387       +4     
==========================================
+ Hits        23039    23040       +1     
- Misses      21084    21086       +2     
- Partials     2260     2261       +1     

Files	Coverage Δ
cmpserver/plugin/plugin.go	80.47% <100.00%> (+0.26%)	⬆️

... and 2 files with indirect coverage changes

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[crenshaw-dev]

Thanks, @peschmae!

[crenshaw-dev]

I think this is a reasonable/safe cherry pick for 2.9.

[crenshaw-dev]

/cherry-pick release-2.9

[crenshaw-dev]

/cherry-pick release-2.8

[gcp-cherry-pick-bot]

Cherry-pick failed with Merge error 752004c8705578b450e04129a13f319a874b940b into temp-cherry-pick-a71f64-release-2.8