add default CODEOWNERS

[jameslamb]

Description

Contributes to rapidsai/build-planning#221

#1054 only touched .clang-format. There's no matching CODEOWNERS rule for that, so no PR reviewer was assigned.

This proposes making rapids-cmake-coedowners the default reviewers for any files not matched by other rules in .github/CODEOWNERS, to avoid that in the future.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.
• The cmake-format.json is up to date with these changes.
• I have added new files under rapids-cmake/
  • I have added include guards (include_guard(GLOBAL))
  • I have added the associated docs/ rst file and update the api.rst

[vyasr]

/merge