feat: Add NVIDIA NeMo datastore (#1852)

# What does this PR do?
Implemetation of NeMO Datastore register, unregister API.

Open Issues: 
- provider_id gets set to `localfs` in client.datasets.register() as it
is specified in routing_tables.py: DatasetsRoutingTable
see: #1860

Currently I have passed `"provider_id":"nvidia"` in metadata and have
parsed that in `DatasetsRoutingTable`
(Not the best approach, but just a quick workaround to make it work for
now.)

## Test Plan
- Unit test cases: `pytest
tests/unit/providers/nvidia/test_datastore.py`
```bash
========================================================== test session starts ===========================================================
platform linux -- Python 3.10.0, pytest-8.3.5, pluggy-1.5.0
rootdir: /home/ubuntu/llama-stack
configfile: pyproject.toml
plugins: anyio-4.9.0, asyncio-0.26.0, nbval-0.11.0, metadata-3.1.1, html-4.1.1, cov-6.1.0
asyncio: mode=strict, asyncio_default_fixture_loop_scope=None, asyncio_default_test_loop_scope=function
collected 2 items                                                                                                                        

tests/unit/providers/nvidia/test_datastore.py ..                                                                                   [100%]

============================================================ warnings summary ============================================================

====================================================== 2 passed, 1 warning in 0.84s ======================================================
```

cc: @dglogo, @mattf, @yanxi0830

tests/integration/providers/nvidia/test_datastore.py

@@ -0,0 +1,47 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+
+
+import pytest
+
+# How to run this test:
+#
+# LLAMA_STACK_CONFIG="nvidia" pytest -v tests/integration/providers/nvidia/test_datastore.py
+
+
+# nvidia provider only
+@pytest.mark.parametrize(
+    "provider_id",
+    [
+        "nvidia",
+    ],
+)
+def test_register_and_unregister(llama_stack_client, provider_id):
+    purpose = "eval/messages-answer"
+    source = {
+        "type": "uri",
+        "uri": "hf://datasets/llamastack/simpleqa?split=train",
+    }
+    dataset_id = f"test-dataset-{provider_id}"
+    dataset = llama_stack_client.datasets.register(
+        dataset_id=dataset_id,
+        purpose=purpose,
+        source=source,
+        metadata={"provider_id": provider_id, "format": "json", "description": "Test dataset description"},
+    )
+    assert dataset.identifier is not None
+    assert dataset.provider_id == provider_id
+    assert dataset.identifier == dataset_id
+
+    dataset_list = llama_stack_client.datasets.list()
+    provider_datasets = [d for d in dataset_list if d.provider_id == provider_id]
+    assert any(provider_datasets)
+    assert any(d.identifier == dataset_id for d in provider_datasets)
+
+    llama_stack_client.datasets.unregister(dataset.identifier)
+    dataset_list = llama_stack_client.datasets.list()
+    provider_datasets = [d for d in dataset_list if d.identifier == dataset.identifier]
+    assert not any(provider_datasets)