Fixes issue

Describe the change

This adds a --external flag to regctl artifact put, aligning it with other regctl artifact commands.

How to verify it

echo "hello external artifact" | regctl artifact put --subject alpine --external ocidir://test/external --artifact-type applicaiton/example.test 

Changelog text

• Feat: Add external flag to regctl artifact put.

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