Create and Publish VerticalManifests during unified builds

[dkurepa]

During unified builds, we run many legs on different OSes. Each of these legs builds multiple repos, and every repo build produces an Asset Manifest, that's later used in the publishing. Based on #4158, we want to use these Asset Manifests to create a Vertical Manifest that will contain information about all assets built in a unified build leg.
We will exclude all of the information about signing, we don't need it

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[dkurepa]

I discussed the implementation details with @ViktorHofer. We'll be adding Publishing.props to the dotnet/dotnet repo that will call a task that will create the VerticalManifest, and initialize a list of files that we want to publish to the remote storage for a given leg

[ViktorHofer]

What I still want to double check with @mmitche is if the task should live in Arcade of the VMR orchestrator. When we invoke the publish action on the VMR we will call into the VMR and will need to make it not again produce a repo manifest. Alternatively if we implement this in Arcade we could add a new mode to the manifest generation that just accepts the other manifest files as an input. I'm not sure which approach is better.

[mmitche]

When we invoke the publish action on the VMR we will call into the VMR and will need to make it not again produce a repo manifest

What do you mean by it producing a repo manifest again?

[ViktorHofer]

@dkurepa, @mmitche and I discussed this offline. Just to add to what @dkurepa mentioned above:

We'll be adding Publishing.props to the dotnet/dotnet repo that will call a task that will create the VerticalManifest, and initialize a list of files that we want to publish to the remote storage for a given leg

@dkurepa given that we won't use Arcade's publishing infrastructure and our current build entry points, it would be easier if we just invoke the merge manifests build task in a target in build.proj that runs AfterTargets="Build" in the VMR orchestrator and write that new file directly into the artifacts folder.