[RFE] Setup CODEOWNERS file

[tormath1]

Current situation

When an external contributor or a maintainer opens a PR, one needs to manually set flatcar-maintainers as reviewer.

Impact

1. It's easy to miss
2. You have to know about the different GitHub teams (nebraska-maintainers, flatcar-maintainers, etc.)

Ideal future situations

There is CODEOWNERS file to automatically set a GitHub team as a reviewer of the PR for a given repo.

Implementation options

• we define one unique CODEOWNERS file here: https://github.com/flatcar/.github
• there is a MAINTAINERS file here: https://github.com/flatcar/Flatcar/blob/main/MAINTAINERS.md - we can decide to maintain both file separately (one is CNCF thing, the other one is a GitHub feature) or find a way to merge them
• flatcar-maintainers should be set as reviewers of all PRs inside the Flatcar organization
• nebraska-maintainers should be set as reviewers of all Nebraska PRs

Additional information

https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-code-owners#about-code-owners

[John15321]

If the CODEOWNERS file is static, maybe with each commit to the main branch, we could have a job that makes sure that the people present in MAINTAINERS.md are also present in CODEOWNERS and if needed appends them?