[Feature Request] Add "copy" functionality

[cmaclaughlin]

The copy function is core to many of us using the ORAS CLI. Theoretically, one could re-create the functionality of copy leveraging a combination of the existing push and pull functions. This adds complexity when the desire is to push related artifacts and, due to the nature of docker push, raises concerns that oras push + oras pull may behave differently than oras copy. As such, the CLI is being invoked within a Python application and the SDK is not used.

Success criteria:

• The ORAS Python client would have a copy function that behaves like the ORAS CLI and includes the recursive copy option.

Note: I haven't spent too much time looking at the code, but I imagine this could be a large effort. And, for us, the work-around of invoking the CLI in code is acceptable. Admittedly, it just isn't desirable.

[cmaclaughlin]

@FeynmanZhou - just FYI