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/distribution/routers/routers.py

@@ -393,3 +393,8 @@ class ToolRuntimeRouter(ToolRuntime):
             tool_id=tool_id,
             args=args,
         )
+
+    async def discover_tools(self, tool_group: ToolGroup) -> List[Tool]:
+        return await self.routing_table.get_provider_impl(
+            tool_group.name
+        ).discover_tools(tool_group)