Add codeowners file for the CNG project

What does this MR do?

Adds a codeowners file for the CNG project.

This includes:

• a new group for project wide maintainers that is @gitlab-org/maintainers/cloud-native-images
• An approval from that group, or delivery or leadership by default for any file
• Required approvals for the codeowners file
• Approvals for dependencies, ci, and the zoekt images that ensure our scoped maintainers can continue to approve and merge for their scoped areas.

Future groups should be created for:

• zoekt dependency maintainers so the dependency maintainers section doesn't get out of hand
• A distribution wide group for CI/CD once we have multiple maintainers that fit that across our distribution projects.

Related issues

Part of gitlab-org/distribution/team-tasks#1253 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• When ready for review, MR is labeled "~workflow::ready for review" per the Distribution MR workflow

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Integration tests added to GitLab QA
• The impact any change in container size has should be evaluated
• New dependencies are managed with dependencies.io