feat(api): add NumPackages to mirrors/repos/snapshots list responses

[lesaux]

Description of the Change

add API response wrappers with NumPackages derived from RefList length; keep show endpoint payloads unchanged for backward compatibility; add API tests for list endpoint NumPackages; update swagger response schemas for list endpoints

Checklist

• allow Maintainers to edit PR (rebase, run coverage, help with tests, ...)
• unit-test added (if change is algorithm)
• functional test added/updated (if change is functional)
• man page updated (if applicable)
• bash completion updated (if applicable)
• documentation updated
• author name in AUTHORS

[codecov]

Codecov Report

❌ Patch coverage is 97.05882% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 76.68%. Comparing base (2ac4c75) to head (c370a48).
⚠️ Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
api/mirror.go	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1559      +/-   ##
==========================================
- Coverage   76.96%   76.68%   -0.28%     
==========================================
  Files         160      161       +1     
  Lines       14757    14779      +22     
==========================================
- Hits        11358    11334      -24     
- Misses       2266     2317      +51     
+ Partials     1133     1128       -5     

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