github: Add CODEOWNERs for component/team reviews

github: Add CODEOWNERs for component/team reviews

Adds [CODEOWNERS](https://help.github.com/en/articles/about-code-owners)
file and assigns ownership of /src/pybind/mgr/dashboard to
@ceph/dashboard alias. That will also automatically add @ceph/dashboard
team to all reviews modifying any file in the above location.

Besides, if "Require review from Code Owners" is marked in the Branch
Protection Rules of any branch, where this CODEOWNERS file is propagated
to, at least one review from a designated CODEOWNER will be mandatory.

If Ceph teams' visibility is changed to public, teams can also be
@mentioned (both from Ceph and other projects), so it's easier to reach
to component-team members.

Fixes: https://tracker.ceph.com/issues/41047
Signed-off-by: Ernesto Puerta <epuertat@redhat.com>