[OpenSSF] Use CODEOWNERS configuration

[Zeitsperre]

In the process of raising our OpenSSF rating, I've stumbled across the feature to have dedicated CODEOWNERS. This provides the ability to specify that certain files or certain folders are identified as the responsibility of particular users. Some good examples of how this could look off the top of my head:

• xclim/indices: huard, tlogan2000, Zeitsperre
• xclim/sdba/*.py: aulemahal, huard
• .github/*/*.yml: Zeitsperre

Specifying these users being the CODEOWNERS would mean that someone listed as an owner must be the reviewer of any Pull Requests affecting those files. This prevents cases where Pull Requests go really far out of scope, as more reviewer approvals would be needed. The category affected is "Branch Protection"

This suggestion comes from OpenSSF. We are in no way obligated to enact this kind of change, but adopting this measure will help raise our rating.

For more info about CODEOWNERS, see: https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-code-owners

[Zeitsperre]

Closing. Not yet relevant for our needs.