Copy API

[shizhMSFT]

Based on #50, the Copy API originated by #8 need to be re-implemented without containerd.

That is, we need our version of

• images.Childen
• images.Dispatch

Since we are implementing the images.Dispatch, we can optimize the graph traversal for our ORAS scenarios.

In fact, images.Dispatch is designed for trees not for DAGs. It is less inefficient if there are multiple nodes pointing to the same node. The pointed node might be transferred multiple times at the same time based on the concurrency level.

In the current code, we also implemented something equivalent to remotes.PushContent. Although the layers are transferred in parallel, the manifests are transferred in sequences. It might not be efficient if we want to copy a manifest list or index.