Add CODEOWNERS file

[RachaelSTamakloe]

This PR adds a CODEOWNERS file to the root of the Toolkit directory. The configuration makes it so that anyone with at least write level permissions is a CODEOWNER. As a result, github automatically requests a review from the relevant parties in the CODEOWNERS file.

Submission Checklist

NOTE: Community submissions can take up to 2 weeks to be reviewed.

Please take the following actions before submitting this pull request.

• Fork your PR branch from the Toolkit "develop" branch (not main)
• Test all changes with pre-commit in a local branch #
• Confirm that "make tests" passes all tests
• Add or modify unit tests to cover code changes
• Ensure that unit test coverage remains above 80%
• Update all applicable documentation
• Follow Cluster Toolkit Contribution guidelines #