rgw: send back ETag on S3 object copy

Fixes: #9479
Backport: firefly, giant
We didn't send the etag back correctly. Original code assumed the etag
resided in the attrs, but attrs only contained request attrs.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
Merge pull request #2865 from ceph/wip-9479

rgw: send back ETag on S3 object copy

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>