Parallelize copy/client.ImageExistsAtRemote calls

[aslafy-z]

When working with a big list of images, getting the remote manifests synchronously can take minutes.
I believe this can easily be parallelized to get some better results.

This is the synchroneous loop that takes time https://github.com/plexsystems/sinker/blob/main/internal/commands/copy.go#L84

[jpreese]

This makes sense to me!