unregister API for dataset (#507)

# What does this PR do?

1) Implement `unregister_dataset(dataset_id)` API in both llama stack
routing table and providers: It removes {dataset_id -> Dataset} mapping
from routing table and removes the dataset_id references in provider as
well (ex. for huggingface, we use a KV store to store the dataset id =>
dataset. we delete it during unregistering as well)

2) expose the datasets/unregister_dataset api endpoint 

## Test Plan

**Unit test:** 

`
pytest llama_stack/providers/tests/datasetio/test_datasetio.py -m
"huggingface" -v -s --tb=short --disable-warnings
`

**Test on endpoint:**
tested llama stack using an ollama distribution template:
1) start an ollama server 
2) Start a llama stack server with the default ollama distribution
config + dataset/datasetsio APIs + datasetio provider
```
---- .../ollama-run.yaml
...
apis:
- agents
- inference
- memory
- safety
- telemetry
- datasetio
- datasets
providers:
  datasetio:
  - provider_id: localfs
    provider_type: inline::localfs
    config: {}
...
```
   saw that the new API showed up in startup script
   
  ```
Serving API datasets
 GET /alpha/datasets/get
 GET /alpha/datasets/list
 POST /alpha/datasets/register
 POST /alpha/datasets/unregister
```

3) query `/alpha/datasets/unregister` through curl (since we have not implemented unregister api in llama stack client)

```
(base) sxyi@sxyi-mbp llama-stack % llama-stack-client datasets register
--dataset-id sixian --url
https://raw.githubusercontent.com/pytorch/torchtune/main/docs/source/tutorials/chat.rst
--schema {}
(base) sxyi@sxyi-mbp llama-stack % llama-stack-client datasets list
┏━━━━━━━━━━━━┳━━━━━━━━━━━━━┳━━━━━━━━━━┳━━━━━━━━━┓
┃ identifier ┃ provider_id ┃ metadata ┃ type    ┃
┡━━━━━━━━━━━━╇━━━━━━━━━━━━━╇━━━━━━━━━━╇━━━━━━━━━┩
│ sixian     │ localfs     │ {}       │ dataset │
└────────────┴─────────────┴──────────┴─────────┘
(base) sxyi@sxyi-mbp llama-stack % llama-stack-client datasets register
--dataset-id sixian2 --url
https://raw.githubusercontent.com/pytorch/torchtune/main/docs/source/tutorials/chat.rst
--schema {}
(base) sxyi@sxyi-mbp llama-stack % llama-stack-client datasets list
┏━━━━━━━━━━━━┳━━━━━━━━━━━━━┳━━━━━━━━━━┳━━━━━━━━━┓
┃ identifier ┃ provider_id ┃ metadata ┃ type    ┃
┡━━━━━━━━━━━━╇━━━━━━━━━━━━━╇━━━━━━━━━━╇━━━━━━━━━┩
│ sixian     │ localfs     │ {}       │ dataset │
│ sixian2    │ localfs     │ {}       │ dataset │
└────────────┴─────────────┴──────────┴─────────┘
(base) sxyi@sxyi-mbp llama-stack % curl
http://localhost:5001/alpha/datasets/unregister \
-H "Content-Type: application/json" \
-d '{"dataset_id": "sixian"}'
null%

(base) sxyi@sxyi-mbp llama-stack % llama-stack-client datasets list
┏━━━━━━━━━━━━┳━━━━━━━━━━━━━┳━━━━━━━━━━┳━━━━━━━━━┓
┃ identifier ┃ provider_id ┃ metadata ┃ type    ┃
┡━━━━━━━━━━━━╇━━━━━━━━━━━━━╇━━━━━━━━━━╇━━━━━━━━━┩
│ sixian2    │ localfs     │ {}       │ dataset │
└────────────┴─────────────┴──────────┴─────────┘
(base) sxyi@sxyi-mbp llama-stack % curl
http://localhost:5001/alpha/datasets/unregister \
-H "Content-Type: application/json" \
-d '{"dataset_id": "sixian2"}'
null%

(base) sxyi@sxyi-mbp llama-stack % llama-stack-client datasets list
```

## Sources


## 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

@@ -81,6 +81,18 @@ class TestDatasetIO:
         assert len(response) == 1
         assert response[0].identifier == "test_dataset"
 
+        with pytest.raises(Exception) as exc_info:
+            # unregister a dataset that does not exist
+            await datasets_impl.unregister_dataset("test_dataset2")
+
+        await datasets_impl.unregister_dataset("test_dataset")
+        response = await datasets_impl.list_datasets()
+        assert isinstance(response, list)
+        assert len(response) == 0
+
+        with pytest.raises(Exception) as exc_info:
+            await datasets_impl.unregister_dataset("test_dataset")
+
     @pytest.mark.asyncio
     async def test_get_rows_paginated(self, datasetio_stack):
         datasetio_impl, datasets_impl = datasetio_stack