feat(api): (1/n) datasets api clean up (#1573)

## PR Stack
- https://github.com/meta-llama/llama-stack/pull/1573
- https://github.com/meta-llama/llama-stack/pull/1625
- https://github.com/meta-llama/llama-stack/pull/1656
- https://github.com/meta-llama/llama-stack/pull/1657
- https://github.com/meta-llama/llama-stack/pull/1658
- https://github.com/meta-llama/llama-stack/pull/1659
- https://github.com/meta-llama/llama-stack/pull/1660

**Client SDK**
- https://github.com/meta-llama/llama-stack-client-python/pull/203

**CI**
- 1391130488
<img width="1042" alt="image"
src="https://github.com/user-attachments/assets/69636067-376d-436b-9204-896e2dd490ca"
/>
-- the test_rag_agent_with_attachments is flaky and not related to this
PR

## Doc
<img width="789" alt="image"
src="https://github.com/user-attachments/assets/b88390f3-73d6-4483-b09a-a192064e32d9"
/>


## Client Usage
```python
client.datasets.register(
    source={
        "type": "uri",
        "uri": "lsfs://mydata.jsonl",
    },
    schema="jsonl_messages",
    # optional 
    dataset_id="my_first_train_data"
)

# quick prototype debugging
client.datasets.register(
    data_reference={
        "type": "rows",
        "rows": [
                "messages": [...],
        ],
    },
    schema="jsonl_messages",
)
```

## Test Plan
- CI:
1387805545

```
LLAMA_STACK_CONFIG=fireworks pytest -v tests/integration/datasets/test_datasets.py
```

```
LLAMA_STACK_CONFIG=fireworks pytest -v tests/integration/scoring/test_scoring.py
```

```
pytest -v -s --nbval-lax ./docs/notebooks/Llama_Stack_Benchmark_Evals.ipynb
```

docs/source/references/evals_reference/index.md

@@ -114,23 +114,17 @@ pprint(response)
 simpleqa_dataset_id = "huggingface::simpleqa"
 
 _ = client.datasets.register(
+    purpose="eval/messages-answer",
+    source={
+        "type": "uri",
+        "uri": "huggingface://datasets/llamastack/simpleqa?split=train",
+    },
     dataset_id=simpleqa_dataset_id,
-    provider_id="huggingface",
-    url={"uri": "https://huggingface.co/datasets/llamastack/simpleqa"},
-    metadata={
-        "path": "llamastack/simpleqa",
-        "split": "train",
-    },
-    dataset_schema={
-        "input_query": {"type": "string"},
-        "expected_answer": {"type": "string"},
-        "chat_completion_input": {"type": "chat_completion_input"},
-    },
 )
 
-eval_rows = client.datasetio.get_rows_paginated(
+eval_rows = client.datasets.iterrows(
     dataset_id=simpleqa_dataset_id,
-    rows_in_page=5,
+    limit=5,
 )
 ```
 
@@ -143,7 +137,7 @@ client.benchmarks.register(
 
 response = client.eval.evaluate_rows(
     benchmark_id="meta-reference::simpleqa",
-    input_rows=eval_rows.rows,
+    input_rows=eval_rows.data,
     scoring_functions=["llm-as-judge::405b-simpleqa"],
     benchmark_config={
         "eval_candidate": {
@@ -191,7 +185,7 @@ agent_config = {
 
 response = client.eval.evaluate_rows(
     benchmark_id="meta-reference::simpleqa",
-    input_rows=eval_rows.rows,
+    input_rows=eval_rows.data,
     scoring_functions=["llm-as-judge::405b-simpleqa"],
     benchmark_config={
         "eval_candidate": {