IPNI Reverse Index for Retrieval Incentives

[masih]

Description: Provide the ability to return a deterministic set of CIDs advertised by a given providerID for a given ContextID.

ETA: 2023-10-31
Maintainer: @masih

### Milestones
- [ ] Design semantics of deterministic CID selection for a given random seed
- [ ] Implement the service
- [ ] Implement observability
- [ ] Deploy to Dev and test integration
- [ ] Production release

Known dependencies:

• Advertisement Mirroring

[xmcai2016]

Watch for updates in the https://github.com/ipni/xedni repo.

[bajtos]

Proposed specification of the new API endpoint: https://github.com/ipni/xedni/blob/master/openapi.yaml