Proposal: Automatically generate Fusion (SxS) manifests for referenced WinMDs' ACIDs

Summary

MSIX tools generate appxmanifest ACIDs (Activatable Class IDs) registrations for referenced WinMDs, but for unpackaged projects there's no such tooling to automate the process, and with the plans to make the dll searching fallback an opt-in feature unpackaged projects will have to either manually add the ACIDs for WinMDs they use to the Fusion manifest which is a tedious process or opt-in to use the dll searching fallback which is slower and increases the binary size, it also doesn't work for Out-of-Process classes, so the best solution here is to provide an opt-out feature to automate the process

Important Notes

There are 2 approaches to achieve this:

• Using mt.exe with the -winmd flag to generate Fusion manifests (Preferred)
• Using a custom MSBuild task that parses the referenced (WindowsMetadataReference items) WinMDs and generate a Fusuion manifest manually containing all classes found in the WinMDs

Open Questions

Should this be implemented in .NET SDK instead?

Additional Notes

I'm willing to implement this feature once approved

Start by comparing the two proposed approaches: invoking mt.exe with the -winmd flag or implementing a custom MSBuild task that parses WindowsMetadataReference items. Resolve whether the feature belongs in CsWinRT or the .NET SDK, then define how referenced WinMD classes should produce Fusion manifests for unpackaged projects.