change schema -> dataset_schema for register_dataset api (#443)

# What does this PR do?

- API updates: change schema to dataset_schema for register_dataset for
resolving pydantic naming conflict
- Note: this OpenAPI update will be synced with
llama-stack-client-python SDK.

cc @dineshyv 

## Test Plan

```
pytest -v -s -m meta_reference_eval_together_inference_huggingface_datasetio eval/test_eval.py
```

## Sources

Please link relevant resources if necessary.


## Before submitting

- [ ] This PR fixes a typo or improves the docs (you can dismiss the
other checks if that's the case).
- [ ] Ran pre-commit to handle lint / formatting issues.
- [ ] Read the [contributor
guideline](https://github.com/meta-llama/llama-stack/blob/main/CONTRIBUTING.md),
      Pull Request section?
- [ ] Updated relevant documentation.
- [ ] Wrote necessary unit or integration tests.

llama_stack/providers/tests/datasetio/test_datasetio.py

@@ -57,7 +57,7 @@ async def register_dataset(
 
     await datasets_impl.register_dataset(
         dataset_id=dataset_id,
-        schema=dataset_schema,
+        dataset_schema=dataset_schema,
         url=URL(uri=test_url),
     )