An S3 object event contains its eTag, currently spelled as 'etag' in the message structure, which does not correspond to the documentation example (https://docs.ceph.com/en/latest/radosgw/notifications/#events), or what is used by AWS (https://docs.aws.amazon.com/AmazonS3/latest/userguide/notification-content-structure.html).
Fixes bug https://tracker.ceph.com/issues/50115
Signed-off-by: Tom Schoonjans <Tom.Schoonjans@rfi.ac.uk>