Feat: Add created timestamp in OCI Layout entries

[sudo-bmitch]

Fixes issue

Describe the change

Add the created timestamp in OCI Layout entries as an annotation. This improves compatibility with other tools and will be useful when the tag listing API is later enhanced by OCI.

How to verify it

regctl image copy alpine:latest ocidir://alpine:latest
jq . <alpine/index.json

Changelog text

• Feat: Add created timestamp in OCI Layout entries.

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

• Tests have been added or not applicable
• Documentation updates are included or not applicable (most documentation should be in the regclient.org repo)
• Changes have been rebased to main
• Multiple commits to the same code have been squashed
• All changes have been human generated or created with a reproducible tool