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/registry/tool_runtime.py

@@ -6,7 +6,13 @@
 
 from typing import List
 
-from llama_stack.distribution.datatypes import Api, InlineProviderSpec, ProviderSpec
+from llama_stack.distribution.datatypes import (
+    AdapterSpec,
+    Api,
+    InlineProviderSpec,
+    ProviderSpec,
+    remote_provider_spec,
+)
 
 
 def available_providers() -> List[ProviderSpec]:
@@ -19,4 +25,13 @@ def available_providers() -> List[ProviderSpec]:
             config_class="llama_stack.providers.inline.tool_runtime.brave_search.config.BraveSearchToolConfig",
             provider_data_validator="llama_stack.providers.inline.tool_runtime.brave_search.BraveSearchToolProviderDataValidator",
         ),
+        remote_provider_spec(
+            api=Api.tool_runtime,
+            adapter=AdapterSpec(
+                adapter_type="model-context-protocol",
+                module="llama_stack.providers.remote.tool_runtime.model_context_protocol",
+                config_class="llama_stack.providers.remote.tool_runtime.model_context_protocol.config.ModelContextProtocolConfig",
+                pip_packages=["mcp"],
+            ),
+        ),
     ]