rgw: Export user stats in get-user-info Adminops API

Fixes: #9359

Signed-off-by: Ray Lv <raylv@yahoo-inc.com>
Merge pull request #2446 from xylv/wip-9359

rgw: Export user stats in get-user-info Adminops API

Reviewed-by: Yehuda Sadeh <yehuda@redhat.com>