Add RID dotnet tool packaging for psign-tool

[mamoreau-devolutions]

Summary

• add RID-specific Devolutions.Psign.Tool dotnet tool packaging for psign-tool
• add NuGet artifact import/pack scripts and local dotnet tool validation helper
• add dry-run CI packaging workflow and manual release workflow with RID-first NuGet publish ordering
• document dotnet tool install and local pack usage

Validation

• cargo check -p psign --bins --lib --locked
• ./nuget/pack-psign-dotnet-tool.ps1 with synthetic six-platform artifacts
• GitHub Actions dry-run workflow dotnet-tool run 25818010855 passed
• Downloaded and inspected dry-run NuGet artifacts; native binaries are placed at tools/net10.0/<rid>/psign-tool(.exe)