add model context protocol provider (#665)

# What does this PR do?
Changes:
* Adds a new API to discover tools available on a runtime
* Adds a new model context protocol provider


## Test Plan

```
# clone python sdk for mcp and start the simple-tool server
uv run mcp-simple-tool --transport sse --port 56000

curl -X POST 'http://localhost:5000/alpha/toolgroups/register' \
-H 'Content-Type: application/json' \
-d '{
  "tool_group": { "name": "simple_mcp_group",
    "type": "model_context_protocol",
    "endpoint": {"uri": "http://localhost:56000/sse"}
  },
  "provider_id": "model-context-protocol"
}'

curl -X POST 'http://localhost:5000/alpha/tool-runtime/invoke' \
-H 'Content-Type: application/json' \
-d '{
    "tool_id": "fetch",
    "args": {
        "url": "http://google.com/"
    }
}'

```

llama_stack/providers/inline/tool_runtime/brave_search/brave_search.py

@@ -4,11 +4,11 @@
 # This source code is licensed under the terms described in the LICENSE file in
 # the root directory of this source tree.
 
-from typing import Any, Dict
+from typing import Any, Dict, List
 
 import requests
 
-from llama_stack.apis.tools import Tool, ToolInvocationResult, ToolRuntime
+from llama_stack.apis.tools import Tool, ToolGroup, ToolInvocationResult, ToolRuntime
 from llama_stack.distribution.request_headers import NeedsRequestProviderData
 from llama_stack.providers.datatypes import ToolsProtocolPrivate
 
@@ -42,6 +42,9 @@ class BraveSearchToolRuntimeImpl(
             )
         return provider_data.api_key
 
+    async def discover_tools(self, tool_group: ToolGroup) -> List[Tool]:
+        raise NotImplementedError("Brave search tool group not supported")
+
     async def invoke_tool(
         self, tool_id: str, args: Dict[str, Any]
     ) -> ToolInvocationResult: