feat: add cd notification commands

[zreigz]

Summary

New commands: plural cd notifications sinks list plural cd notifications sinks upsert NAME

Labels

Test Plan

Checklist

• [ ] If required, I have updated the Plural documentation accordingly.
• [ ] I have added tests to cover my changes.
• [ ] I have added a meaningful title and summary to convey the impact of this PR to a user.
• [ ] I have added relevant labels to this PR to help with categorization for release notes.