Add `CODEOWNERS` file to auto-request reviews

[timvaillancourt]

A Pull Request should be associated with an Issue.

Related issue: N/A

Further notes in https://github.com/github/gh-ost/blob/master/.github/CONTRIBUTING.md Thank you! We are open to PRs, but please understand if for technical reasons we are unable to accept each and any PR

Description

This PR adds a .github/CODEOWNERS file so reviews from the usual maintainers is auto-requested when a PR is made. Feel free to add who is missing

In case this PR introduced Go code changes:

• [x] contributed code is using same conventions as original code
• [x] script/cibuild returns with no formatting errors, build errors or unit test errors.

[timvaillancourt]

Ping @shlomi-noach in case you want to review PRs