rgw/notify: visit() returns copy of owner string

rgw/notify: visit() returns copy of owner string

return a copy of the owner string to work around compiler warning:

ceph/src/rgw/driver/rados/rgw_notify.cc: In function ‘int rgw::notify::publish_reserve(const DoutPrefixProvider*, const rgw::SiteConfig&, const EventTypeList&, reservation_t&, const RGWObjTags*)’:
ceph/src/rgw/driver/rados/rgw_notify.cc:1113:26: warning: possibly dangling reference to a temporary [-Wdangling-reference]
 1113 |       const std::string& topic_tenant = std::visit(fu2::overload(
      |                          ^~~~~~~~~~~~
ceph/src/rgw/driver/rados/rgw_notify.cc:1113:51: note: the temporary was destroyed at the end of the full expression ‘std::visit<fu2::abi_310::detail::overloading::overload_impl<rgw::notify::publish_reserve(const DoutPrefixProvider*, const rgw::SiteConfig&, const EventTypeList&, reservation_t&, const RGWObjTags*)::<lambda(const rgw_user&)>, rgw::notify::publish_reserve(const DoutPrefixProvider*, const rgw::SiteConfig&, const EventTypeList&, reservation_t&, const RGWObjTags*)::<lambda(const rgw_account_id&)> >, variant<rgw_user, rgw_account_id>&>(fu2::overload<rgw::notify::publish_reserve(const DoutPrefixProvider*, const rgw::SiteConfig&, const EventTypeList&, reservation_t&, const RGWObjTags*)::<lambda(const rgw_user&)>, rgw::notify::publish_reserve(const DoutPrefixProvider*, const rgw::SiteConfig&, const EventTypeList&, reservation_t&, const RGWObjTags*)::<lambda(const rgw_account_id&)> >(<lambda closure object>rgw::notify::publish_reserve(const DoutPrefixProvider*, const rgw::SiteConfig&, const EventTypeList&, reservation_t&, const RGWObjTags*)::<lambda(const rgw_user&)>(), <lambda closure object>rgw::notify::publish_reserve(const DoutPrefixProvider*, const rgw::SiteConfig&, const EventTypeList&, reservation_t&, const RGWObjTags*)::<lambda(const rgw_account_id&)>()), topic_cfg.rgw_pubsub_topic::owner)’
 1113 |       const std::string& topic_tenant = std::visit(fu2::overload(
      |                                         ~~~~~~~~~~^~~~~~~~~~~~~~~
 1114 |           [] (const rgw_user& u) -> const std::string& { return u.tenant; },
      |           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 1115 |           [] (const rgw_account_id& a) -> const std::string& { return a; }
      |           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 1116 |           ), topic_cfg.owner);
      |           ~~~~~~~~~~~~~~~~~~~

Fixes: https://tracker.ceph.com/issues/67326
Signed-off-by: Casey Bodley <cbodley@redhat.com>