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.

docs/resources/llama-stack-spec.html

@@ -2291,6 +2291,39 @@
                     "required": true
                 }
             }
+        },
+        "/alpha/datasets/unregister": {
+            "post": {
+                "responses": {
+                    "200": {
+                        "description": "OK"
+                    }
+                },
+                "tags": [
+                    "Datasets"
+                ],
+                "parameters": [
+                    {
+                        "name": "X-LlamaStack-ProviderData",
+                        "in": "header",
+                        "description": "JSON-encoded provider data which will be made available to the adapter servicing the API",
+                        "required": false,
+                        "schema": {
+                            "type": "string"
+                        }
+                    }
+                ],
+                "requestBody": {
+                    "content": {
+                        "application/json": {
+                            "schema": {
+                                "$ref": "#/components/schemas/UnregisterDatasetRequest"
+                            }
+                        }
+                    },
+                    "required": true
+                }
+            }
         }
     },
     "jsonSchemaDialect": "https://json-schema.org/draft/2020-12/schema",
@@ -7917,6 +7950,18 @@
                 "required": [
                     "model_id"
                 ]
+            },
+            "UnregisterDatasetRequest": {
+                "type": "object",
+                "properties": {
+                    "dataset_id": {
+                        "type": "string"
+                    }
+                },
+                "additionalProperties": false,
+                "required": [
+                    "dataset_id"
+                ]
             }
         },
         "responses": {}
@@ -8529,6 +8574,10 @@
             "name": "UnregisterModelRequest",
             "description": "<SchemaDefinition schemaRef=\"#/components/schemas/UnregisterModelRequest\" />"
         },
+        {
+            "name": "UnregisterDatasetRequest",
+            "description": "<SchemaDefinition schemaRef=\"#/components/schemas/UnregisterDatasetRequest\" />"
+        },
         {
             "name": "UnstructuredLogEvent",
             "description": "<SchemaDefinition schemaRef=\"#/components/schemas/UnstructuredLogEvent\" />"
@@ -8718,6 +8767,7 @@
                 "URL",
                 "UnregisterMemoryBankRequest",
                 "UnregisterModelRequest",
+                "UnregisterDatasetRequest",
                 "UnstructuredLogEvent",
                 "UserMessage",
                 "VectorMemoryBank",