Add "s3/"-prefix to S3 destination.service.resource

# Description
Implementation issue for spec change elastic/apm#738.
The spec for the `context.destination.service.resource` span field for AWS S3 has been changed from `${bucketName}` to `s3/{bucketName}`.

This change should be classified as a bugfix in the agent release notes and the release notes should state that this change is potentially breaking:
* It can break the history of the S3-Spans / metrics for users relying on `context.destination.service.resource`
* If users happen to run agents both with and without this fix (for same or different languages), the same S3-buckets can appear twice in the service map (with and without s3-prefix)

## Agent Issues
- [ ] https://github.com/elastic/apm-agent-java/issues/2946
- [ ] https://github.com/elastic/apm-agent-nodejs/issues/3084
- [ ] https://github.com/elastic/apm-agent-python/issues/1717
- [ ] https://github.com/elastic/apm-agent-go/issues/1367
- [ ] https://github.com/elastic/apm-agent-ruby/issues/1337