support extended copying

[qweeah]

As an extension of https://github.com/oras-project/oras/issues/826, add a new flag --extended to support extended copying. With same content layout described in #826, the extended copying command is expected to have behavior:

• oras cp -r [--from-oci-layout] --extended $FROM:v1 $TO:v2: copy ML1, A1, M1, M2 and M3 to $TO and tag ML1 as v2.
• oras cp -r [--from-oci-layout] --extended $FROM:v1 --platform linux/amd64 $TO:v2 or oras cp -r [--from-oci-layout] $FROM@$M1_DIGEST $TO:v2: copy ML1, A1, M1, A2, M2 and M3 to $TO and tag M1 as v2

Reference: https://oras.land/client_libraries/#extendedcopy

[github-actions[bot]]

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