Fixes issue

Fixes #760.

Describe the change

This adds an experimental regctl ref command to parse image references and allow formatted output. This also cleans up the digest command a bit (using cobra for input vs os.Stdin, and templating the output).

How to verify it

regctl ref nginx --format '{{.Registry}}'

Changelog text

• Add an experimental regctl ref command.

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