Support Object Composition

**Is your feature request related to a problem or challenge? Please describe what you are trying to do.**

Various stores have facilities for object composition:

* https://docs.aws.amazon.com/AmazonS3/latest/API/API_UploadPartCopy.html
* https://cloud.google.com/storage/docs/xml-api/put-object-compose
* https://learn.microsoft.com/en-us/rest/api/storageservices/put-block-from-url

**Describe the solution you'd like**

Not sure, but documenting that this is something supported by the stores but not the `ObjectStore` trait

**Describe alternatives you've considered**

**Additional context**

Start by reviewing the ObjectStore trait and the object-composition API references for AWS S3, Google Cloud Storage, and Microsoft storage. Clarify the intended cross-store behavior and API shape before implementation; done should include an agreed trait design and support for the relevant stores.

Written by the indexing model from the issue text.