Design how we would support idempotency of create/add pieces

## Done Criteria
A "design" (likely comments in this issue) covers the content needed for a "support idempotency of create/add pieces" issue and that issue and the related issues have been created.

The design should cover:
1. What problem we're trying to solve / why it's important
2. How we expect this to be exposed to users in Synapse
3. Other implementation details (e.g., what's the idempotency key, what HTTP status code will Curio respond with)
4. Quick comparison of other prior art for this in FOC or other cloud providers
5. Breakdown of the tasks across Curio, Synapse, Docs, etc.

## Why Important
This is a feature that touches a couple of areas of the repo and needs to be documented to users. It should be looked at (and prioritized) holistically.

## Notes
* Some initial Curio implementation work with some discussion was previously started in https://github.com/filecoin-project/curio/pull/813