Add mount support for oras cp

[ktarplee]

What is the version of your ORAS CLI

main

What would you like to be added?

When the source and destination registries are the same, the code needs to simply needs to tell oras-go via the oras.CopyOptions that we want to mount from the source repository if available.

An example of how to do this is here. You need to implement the MountFrom function and have it return the source repository (if the registry is the same as the destination). You probably also want to implement OnMount.

Why is this needed for ORAS?

There is a large performance improvement possible if mounting is supported. crane cp has always supported mounting and now that oras-go supports mounting (via PR), we should add it to oras cp.

Are you willing to submit PRs to contribute to this feature?

• Yes, I am willing to implement it.