add useragent in registry config to send it to s3
To help tracing of requests in s3 (eg #63) we can send in the useragent "registry-prod" and "registry-staging". This wills show up the the logs of s3.
https://github.com/distribution/distribution/blob/main/docs/content/storage-drivers/s3.md
it's not suppored by harbor's chart at the moment: https://github.com/goharbor/harbor-helm/blob/main/templates/registry/registry-cm.yaml#L49