fix: warning message is printed to stdout by CLI

[susanshi]

Description

What this PR does / why we need it:

Currently we are printing warning log out to stdout which pollutes the json verification output.
This PR contains following update:

• Warning should be logged using the logger.
• added a unit test file for cli cmd. However the cli command currently is not unit testable, the test currently only validates the command exist.

Which issue(s) this PR fixes (optional, using fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when the PR gets merged):

Fixes #1565

Type of change

Please delete options that are not relevant.

• [X ] Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Validated the output only contains the verification result json. The warning is now logged ot stderr.

Checklist:

• Does the affected code have corresponding tests?
• Are the changes documented, not just with inline documentation, but also with conceptual documentation such as an overview of a new feature, or task-based documentation like a tutorial? Consider if this change should be announced on your project blog.
• Does this introduce breaking changes that would require an announcement or bumping the major version?
• Do all new files have appropriate license header?

Post Merge Requirements

• MAINTAINERS: manually trigger the "Publish Package" workflow after merging any PR that indicates Helm Chart Change

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files	Coverage Δ
cmd/ratify/cmd/discover.go	32.09% <100.00%> (+13.58%)	⬆️
cmd/ratify/cmd/referrer.go	49.15% <100.00%> (+16.94%)	⬆️
cmd/ratify/cmd/verify.go	75.00% <100.00%> (+8.92%)	⬆️
internal/logger/logger.go	100.00% <ø> (ø)

[junczhu]

LGTM

[susanshi]

todo: Add issue for improve unit test for cli

[binbin-li]

todo: Add issue for improve unit test for cli

Can you add the reference to notation CLI test in the issue? https://github.com/notaryproject/notation/tree/main/cmd/notation
Adding unit test for CMDs could be quite effort-intensive.

[binbin-li]

lgtm!