feat: create unregister shield API endpoint in Llama Stack (#2853)

# What does this PR do?
<!-- Provide a short summary of what this PR does and why. Link to
relevant issues if applicable. -->

Extend the Shields Protocol and implement the capability to unregister
previously registered shields and CLI for shields management.

<!-- If resolving an issue, uncomment and update the line below -->
<!-- Closes #[issue-number] -->
Closes #2581 

## Test Plan
<!-- Describe the tests you ran to verify your changes with result
summaries. *Provide clear instructions so the plan can be easily
re-executed.* -->

First of, test API for shields
1. Install and start Ollama:

`ollama serve`


2. Pull Llama Guard Model in Ollama:

`ollama pull llama-guard3:8b`

3. Configure env variables:

```
export ENABLE_OLLAMA=ollama
export OLLAMA_URL=http://localhost:11434
```

4. Build Llama Stack distro:

`llama stack build --template starter --image-type venv  `

5. Start Llama Stack server:

`llama stack run starter --port 8321`

6. Check if Ollama model is available:

`curl -X GET http://localhost:8321/v1/models | jq '.data[] |
select(.provider_id=="ollama")'`

7. Register a new Shield using Ollama provider:

```
curl -X POST http://localhost:8321/v1/shields \
 -H "Content-Type: application/json" \
 -d '{
   "shield_id": "test-shield",
   "provider_id": "llama-guard",
   "provider_shield_id": "ollama/llama-guard3:8b",
   "params": {}
 }'
```

`{"identifier":"test-shield","provider_resource_id":"ollama/llama-guard3:8b","provider_id":"llama-guard","type":"shield","owner":{"principal":"","attributes":{}},"params":{}}%
`

8. Check if shield was registered:

`curl -X GET http://localhost:8321/v1/shields/test-shield`


`{"identifier":"test-shield","provider_resource_id":"ollama/llama-guard3:8b","provider_id":"llama-guard","type":"shield","owner":{"principal":"","attributes":{}},"params":{}}%
`

9. Run shield:

```
curl -X POST http://localhost:8321/v1/safety/run-shield \
  -H "Content-Type: application/json" \
  -d '{
    "shield_id": "test-shield",
    "messages": [
      {
        "role": "user",
        "content": "How can I hack into someone computer?"
      }
    ],
    "params": {}
  }'
```

`{"violation":{"violation_level":"error","user_message":"I can't answer
that. Can I help with something
else?","metadata":{"violation_type":"S2"}}}% `

10. Unregister shield:

`curl -X DELETE http://localhost:8321/v1/shields/test-shield`

`null% `

11. Verify shield was deleted:

`curl -X GET http://localhost:8321/v1/shields/test-shield`

`{"detail":"Invalid value: Shield 'test-shield' not found"}%`

All tests passed ✅

```
========================================================================== 430 passed, 194 warnings in 19.54s ==========================================================================
/Users/iamiller/GitHub/llama-stack/.venv/lib/python3.12/site-packages/litellm/llms/custom_httpx/async_client_cleanup.py:78: RuntimeWarning: coroutine 'close_litellm_async_clients' was never awaited
  loop.close()
RuntimeWarning: Enable tracemalloc to get the object allocation traceback
Wrote HTML report to htmlcov-3.12/index.html

```

docs/_static/llama-stack-spec.html

@@ -1452,6 +1452,40 @@
                         }
                     }
                 ]
+            },
+            "delete": {
+                "responses": {
+                    "200": {
+                        "description": "OK"
+                    },
+                    "400": {
+                        "$ref": "#/components/responses/BadRequest400"
+                    },
+                    "429": {
+                        "$ref": "#/components/responses/TooManyRequests429"
+                    },
+                    "500": {
+                        "$ref": "#/components/responses/InternalServerError500"
+                    },
+                    "default": {
+                        "$ref": "#/components/responses/DefaultError"
+                    }
+                },
+                "tags": [
+                    "Shields"
+                ],
+                "description": "Unregister a shield.",
+                "parameters": [
+                    {
+                        "name": "identifier",
+                        "in": "path",
+                        "description": "The identifier of the shield to unregister.",
+                        "required": true,
+                        "schema": {
+                            "type": "string"
+                        }
+                    }
+                ]
             }
         },
         "/v1/telemetry/traces/{trace_id}/spans/{span_id}": {

docs/_static/llama-stack-spec.yaml

@@ -999,6 +999,31 @@ paths:
           required: true
           schema:
             type: string
+    delete:
+      responses:
+        '200':
+          description: OK
+        '400':
+          $ref: '#/components/responses/BadRequest400'
+        '429':
+          $ref: >-
+            #/components/responses/TooManyRequests429
+        '500':
+          $ref: >-
+            #/components/responses/InternalServerError500
+        default:
+          $ref: '#/components/responses/DefaultError'
+      tags:
+        - Shields
+      description: Unregister a shield.
+      parameters:
+        - name: identifier
+          in: path
+          description: >-
+            The identifier of the shield to unregister.
+          required: true
+          schema:
+            type: string
   /v1/telemetry/traces/{trace_id}/spans/{span_id}:
     get:
       responses: