Extend validate-rc-by-pmc command to support providers validation

[shahar1]

• Add ProvidersReleaseValidator class for validating provider release candidates
• Update validate-rc-by-pmc command to support --distribution providers
• Add --release-date and --packages-file options for providers validation
• Implement all required validation checks: SVN files, reproducible build, signatures, checksums, licenses
• Parse packages.txt file containing PyPI URLs from VOTE email
• Support building and comparing provider distributions with SVN artifacts

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Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)
  Claude Opus 4.5 + Gemini 3 Pro

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[shahar1]

Closing for now as focus shifts to ATR.
If we need it as a fallback, I'll revive it.