CODEOWNERS in specs repo

Add CODEOWNERS validation to specs repo.

Initial pieces discussed include:

1. Add CODEOWNERS linter and set up a baseline (this baseline may drift over time as organizations change)
2. In PRs
1. Lint CODEOWNERS associated with changed files only and fail if the associated CODEOWNERS are not valid so that the PR contributor can update the CODEOWNERS file before merging
2. Fail if there are no CODEOWNERS for any file in the PR, require the opener to update CODEOWNERS