Add /sse/hub-cache to update the Hub's backend cache
TODO:

- [x] setup replicaset in mongo server, because it's required to be able to watch the collection changes
- [x] feature: send an SSE on every update of a `dataset-hub-cache` cache entry
- [x] feature: ^ only if some value has changed (?)
- [x] feature: On the first call to /sse/hub-cache, emit one SSE per `dataset-hub-cache` cache entry (initial synchronization)
- [x] <strike>document in OpenAPI (no need to add to the docs, as it's internal)</strike> -> No: it's not supported by the OpenAPI specification (see https://github.com/OAI/OpenAPI-Specification/issues/396)
- [x] add sseApi in https://github.com/huggingface/infra-deployments/blob/main/apps/datasets-server/datasets-server-prod.yaml and https://github.com/huggingface/infra-deployments/blob/main/apps/datasets-server/datasets-server-dev.yaml (internal)?
- [x] document the usage of /sse/hub-cache in `services/sse-api/README.md` since it's not possible to do so in the OpenAPI spec

Maybe:
- [ ] add e2e tests
- [ ] monitor the overhead that it generates?
- [ ] How to make features 1 and 3 work well together? What if a `dataset-hub-cache` cache entry is updated in the middle of the initial synchronization SSE? How do we test this?
- [ ] protect the endpoint so only the Hub can request it?

Add /sse/hub-cache to update the Hub's backend cache: TODO:

- [x] setup replicaset in mongo server, because it's required to be able to watch the collection changes
- [x] feature: send an SSE on every update of a `dataset-hub-cache` cache entry
- [x] feature: ^ only if some value has changed (?)
- [x] feature: On the first call to /sse/hub-cache, emit one SSE per `dataset-hub-cache` cache entry (initial synchronization)
- [x] <strike>document in OpenAPI (no need to add to the docs, as it's internal)</strike> -> No: it's not supported by the OpenAPI specification (see https://github.com/OAI/OpenAPI-Specification/issues/396)
- [x] add sseApi in https://github.com/huggingface/infra-deployments/blob/main/apps/datasets-server/datasets-server-prod.yaml and https://github.com/huggingface/infra-deployments/blob/main/apps/datasets-server/datasets-server-dev.yaml (internal)?
- [x] document the usage of /sse/hub-cache in `services/sse-api/README.md` since it's not possible to do so in the OpenAPI spec

Maybe:
- [ ] add e2e tests
- [ ] monitor the overhead that it generates?
- [ ] How to make features 1 and 3 work well together? What if a `dataset-hub-cache` cache entry is updated in the middle of the initial synchronization SSE? How do we test this?
- [ ] protect the endpoint so only the Hub can request it?
