feat(cli): add a --validate option in argo CLI (#23846)

[chansuke]

Closes #23846

Checklist:

• 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.
• The title of the PR states what changed and the related issues number (used for the release note).
• The title of the PR conforms to the Toolchain Guide
• 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.
• Does this PR require documentation updates?
• I've updated documentation as required by this PR.
• 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.
• I have added a brief description of why this PR is necessary and/or what this PR solves.
• Optional. My organization is added to USERS.md.
• Optional. For bug fixes, I've indicated what older releases this fix should be cherry-picked into (this may or may not happen depending on risk/complexity).

[bunnyshell]

❌ Preview Environment undeployed from Bunnyshell

Available commands (reply to this comment):

• 🚀 /bns:deploy to deploy the environment

[codecov]

Codecov Report

❌ Patch coverage is 32.25806% with 21 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.21%. Comparing base (660295f) to head (dbc9fd0).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
cmd/argocd/commands/app.go	32.25%	21 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #23949      +/-   ##
==========================================
- Coverage   60.21%   60.21%   -0.01%     
==========================================
  Files         346      346              
  Lines       59268    59295      +27     
==========================================
+ Hits        35690    35705      +15     
- Misses      20717    20735      +18     
+ Partials     2861     2855       -6     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[pjiang-dev]

@chansuke Thanks for the PR. Just FYI this is the server-side diff CLI PR that is not merged yet https://github.com/argoproj/argo-cd/pull/23978/files#diff-18d6bf6e8588f888e23c671094d44b6bba958c624dccb0f734f40cfe91b2f1f0

You will probably pass your validate flag to my gRPC which will then use validate=true when doing server-side diff in gitops-engine, which will also need a few changes to accept that flag, as right now it is hardcoded to false.