datumctl has no third-party license notice

[scotwells]

Summary

datumctl ships binaries for Linux, macOS and Windows that bundle about 140 third-party Go packages, but the repository has no NOTICE file listing their licenses. Other Datum repositories keep one, and the release process expects it to be current before each tag.

Acceptance criteria

• The repository has a NOTICE file covering the dependencies of every platform the release builds
• A maintainer can regenerate it with one task after dependencies change