github: rgw team and CODEOWNERS

i've created an rgw team [1] on github for use with CODEOWNERS [2]. i also raised a pull request [3] to add rgw paths to our .github/CODEOWNERS file members of the team will get github notifications to review pull requests that touch any of these paths. this can be spammy, so i've only added the members listed in src/rgw/MAINTAINERS.md. if other devs would like to opt-in to this team, please reach out to me off-list [1] https://github.com/orgs/ceph/teams/rgw [2] https://docs.github.com/en/repositories/managing-your-repositorys-settings-… [3] https://github.com/ceph/ceph/pull/50073