feat: add ConfigHub configuration provider

[windischb]

Summary

• add the dedicated Cocoar.Configuration.ConfigHub NuGet package
• implement authenticated snapshots, ETag refresh, SSE invalidation, reconnect reconciliation, and polling fallback
• add 18 focused provider, resilience, security, and dynamic-rule tests
• integrate package metadata, solution wiring, docs, roadmap, and changelog

Verification

• dotnet build src/Cocoar.Configuration.slnx -c Release
• dotnet test -c Release --no-build --no-restore --filter "Type!=Performance" (797 passed, 4 skipped)
• dotnet pack including symbols; 12 package pairs generated
• VitePress build
• TypeScript build and tests
• NuGet vulnerability scan: clean

Versioning

+semver: minor — intended for the 6.2.0 line and a 6.2.0-beta.* prerelease after merge.