Add workers flag to validate input command

[robnester-rh]

This commit reworks the concurrency model of validate input to be like that found in validate image. This commit adds a --workers flag, with a default of 5 that can be used to configure how many simultaneous fetches occur. This avoids situations where validating multiple input files can trigger HTTP 429 errors.

[codecov]

Codecov Report

Attention: Patch coverage is 91.89189% with 3 lines in your changes missing coverage. Please review.

Project coverage is 72.42%. Comparing base (9110d5f) to head (9abf08b).
Report is 12 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/validate/input.go	91.89%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2218      +/-   ##
==========================================
+ Coverage   71.19%   72.42%   +1.22%     
==========================================
  Files          88       88              
  Lines        7520     7539      +19     
==========================================
+ Hits         5354     5460     +106     
+ Misses       2166     2079      -87     

Flag	Coverage Δ
generative	72.42% <91.89%> (+1.22%)	⬆️
integration	72.42% <91.89%> (+1.22%)	⬆️
unit	72.42% <91.89%> (+1.22%)	⬆️

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Files with missing lines	Coverage Δ
cmd/validate/input.go	89.80% <91.89%> (+47.55%)	⬆️

[zregvart]

Thanks for adding tests. 👍 LGTM