Fixes issue

Describe the change

Add the ability to copy images with external source and/or target referrer repositories.

How to verify it

echo "hello external artifact" | regctl artifact put --subject alpine --external ocidir://test/external --artifact-type application/example.test
regctl image copy alpine localhost:5002/library/alpine --referrers --referrers-src ocidir://test/external --referrers-tgt localhost:5002/external

Changelog text

• Feat: Copy image with external referrers.

Please verify and check that the pull request fulfills the following requirements

• Tests have been added or not applicable
• Documentation has been added, updated, or not applicable
• Changes have been rebased to main
• Multiple commits to the same code have been squashed