chore: Add CODEOWNERS

What does this MR do?

Adds missing CODEOWNERS including @rossfuhrman and @theoretick

At any time, the manager/mentor may choose to open a merge request, adding the reviewer as a maintainer. This merge request should have a justification from the manager as to why the reviewer should become a maintainer. You are also welcome to open this merge request yourself at any time.
https://about.gitlab.com/handbook/engineering/workflow/code-review/#how-to-become-a-project-maintainer

@rossfuhrman has demonstrated consistent domain knowledge and maintainer-level commitments to quality and consistency across the product. This includes regular releases (covering community guidance and contributions), updates to our workflow guidance, and proposals to further align our release process with existing paradigms across the group.