feat(api): (1.2/n) datasets.iterrorws pagination api updates (#1656)

# What does this PR do?
- as title
- uses "cursor" pagination scheme for iterrows

[//]: # (If resolving an issue, uncomment and update the line below)
[//]: # (Closes #[issue-number])

## Test Plan
<img width="1226" alt="image"
src="https://github.com/user-attachments/assets/3220eaac-7117-4d0a-b344-2bbb77a22065"
/>


[//]: # (## Documentation)

llama_stack/apis/datasetio/datasetio.py

@@ -13,19 +13,16 @@ from llama_stack.schema_utils import json_schema_type, webmethod
 
 
 @json_schema_type
-class PaginatedRowsResult(BaseModel):
+class IterrowsResponse(BaseModel):
     """
     A paginated list of rows from a dataset.
 
-    :param rows: The rows in the current page.
-    :param total_count: The total number of rows in the dataset.
-    :param next_page_token: The token to get the next page of rows.
+    :param data: The rows in the current page.
+    :param next_index: Index into dataset for the first row in the next page. None if there are no more rows.
     """
 
-    # the rows obey the DatasetSchema for the given dataset
-    rows: List[Dict[str, Any]]
-    total_count: int
-    next_page_token: Optional[str] = None
+    data: List[Dict[str, Any]]
+    next_index: Optional[int] = None
 
 
 class DatasetStore(Protocol):
@@ -41,18 +38,18 @@ class DatasetIO(Protocol):
     async def iterrows(
         self,
         dataset_id: str,
-        rows_in_page: int,
-        page_token: Optional[str] = None,
-        filter_condition: Optional[str] = None,
-    ) -> PaginatedRowsResult:
-        """Get a paginated list of rows from a dataset.
+        start_index: Optional[int] = None,
+        limit: Optional[int] = None,
+    ) -> IterrowsResponse:
+        """Get a paginated list of rows from a dataset. Uses cursor-based pagination.
 
         :param dataset_id: The ID of the dataset to get the rows from.
-        :param rows_in_page: The number of rows to get per page.
-        :param page_token: The token to get the next page of rows.
-        :param filter_condition: (Optional) A condition to filter the rows by.
+        :param start_index: Index into dataset for the first row to get. Get all rows if None.
+        :param limit: The number of rows to get per page.
         """
         ...
 
-    @webmethod(route="/datasets/{dataset_id}/rows", method="POST")
-    async def append_rows(self, dataset_id: str, rows: List[Dict[str, Any]]) -> None: ...
+    @webmethod(route="/datasets/{dataset_id}/append-rows", method="POST")
+    async def append_rows(
+        self, dataset_id: str, rows: List[Dict[str, Any]]
+    ) -> None: ...