Implement export of OCI Image Manifests as Tarballs

[baronfel]

In dotnet/sdk#43085 we implemented multi-arch image publishing to registries, with tarball exports of the single images as Docker-formatted tarballs. We should instead export these as OCI Image Manifest tarballs so that annotations and platform metadata can be preserved in the single unit of transfer. While this can be stitched back together, there is risk of data loss and the UX is not great.

Most tools (docker/podman/etc) understand this format today (Docker as long as the containerd backend is in use, which is the default).

Spec

When

• a project is being targeted for multiple RuntimeIdentifiers, AND
• the project is being exported to a local directory, THEN
• instead of exporting a Docker tarball layout per-RID (today's behavior) we should output a single, multi-architecture OCI Image Manifest.

The format of the tarball should be as described in the Image Layout specification.