Cross Bucket Copy Support

_Draft as needs further design, and a motivating use-case_

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

Currently ObjectStore only supports copying files within the current bucket, this is despite most stores having APIs for performing efficient copies across buckets within the same cloud provider.

**Describe the solution you'd like**

I would like some mechanism to support copying between ObjectStore, that is able to make use of the more efficient APIs where they are possible. I don't have a good idea of how to achieve this, nor a motivating use-case for this functionality.

**Describe alternatives you've considered**

**Additional context**

Start with the existing ObjectStore copy operation and compare how supported stores expose same-provider cross-bucket copy APIs. Define a motivating use case and settle the API and design boundaries; done requires an agreed design that can express cross-bucket copies while preserving efficient provider-side operations where available.

Written by the indexing model from the issue text.