Add a CODEOWNERS file to auto-assign main key holders as reviewers
This helps in auto-assigning reviewers and requiring approvals by the group of main key holders @archlinux/teams/main-key-holders.
It leaves out the Python sources to require two approvals, as those have moved to the keyringctl repository and will soon be removed from this repository.
@anthraxx @artafinde since you are in the devops group and can set these things, please make sure, that we are setup correctly: https://docs.gitlab.com/ee/user/project/codeowners/#require-multiple-approvals-from-code-owners