[swift][s3api] Same bucket object storage policy

Hey there!

We would very much like to allow multiple objects in the same bucket to have multiple storage policies, different from their container. Since we noticed that this is not possible now, we'd like to take a shot at it. But first, any suggestions or ideas on implementing this?

The idea we are following here and trying to implement would be to use the same logic as the `+segments` bucket: each bucket would have a `+cold` counterpart, in the cold storage policy, which would store such objects, and we would keep a manifest/symlink in the original bucket, which we can use to retrieve the content from cold storage.

Just as a piece of context, we are basing this solution on yoga release and using S3 apis mainly.

Thanks for the help! Rafael