Direct SMTP delivery to storage.blob (filesystem, S3)

There is storage.blob module namespace that is used by IMAP storage layer to store message bodies.
It would be helpful if we can implement an adapter that would allow SMTP pipeline to directly write messages
into blob storage (which can be backed by FS or S3 bucket or something else).

I guess we could just make all storage.blob implementations also implement delivery target interface via some helper utility
so users can just write:
```
deliver_to s3 { bucket config goes here }
```

Important details:
1. How object (file) name should be formatted?
2. How SMTP metadata should be saved (if saved at all)?