Tools API with brave and MCP providers (#639)

This PR adds a new Tools api and adds two tool runtime providers: brave
and MCP.

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

 curl -X POST 'http://localhost:5000/alpha/toolgroups/register' \
-H 'Content-Type: application/json' \
-d '{
  "tool_group_id": "search", "provider_id": "brave-search",
  "tool_group": {
    "type": "user_defined",
    "tools": [
      {
        "name": "brave_search",
        "description": "A web search tool",
        "parameters": [
          {
            "name": "query",
            "parameter_type": "string",
            "description": "The query to search"
          }
        ],
        "metadata": {},
        "tool_prompt_format": "json"
      }
    ]
  }
}'

 curl -X GET http://localhost:5000/alpha/tools/list | jq .
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   662  100   662    0     0   333k      0 --:--:-- --:--:-- --:--:--  646k
[
  {
    "identifier": "brave_search",
    "provider_resource_id": "brave_search",
    "provider_id": "brave-search",
    "type": "tool",
    "tool_group": "search",
    "description": "A web search tool",
    "parameters": [
      {
        "name": "query",
        "parameter_type": "string",
        "description": "The query to search"
      }
    ],
    "metadata": {},
    "tool_prompt_format": "json"
  },
  {
    "identifier": "fetch",
    "provider_resource_id": "fetch",
    "provider_id": "model-context-protocol",
    "type": "tool",
    "tool_group": "simple_tool",
    "description": "Fetches a website and returns its content",
    "parameters": [
      {
        "name": "url",
        "parameter_type": "string",
        "description": "URL to fetch"
      }
    ],
    "metadata": {
      "endpoint": "http://localhost:56000/sse"
    },
    "tool_prompt_format": "json"
  }
]

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

 curl -X POST 'http://localhost:5000/alpha/tool-runtime/invoke' \
-H 'Content-Type: application/json' -H 'X-LlamaStack-ProviderData: {"api_key": "<KEY>"}' \
-d '{
    "tool_name": "brave_search",
    "args": {
        "query": "who is meta ceo"
    }
}'
```

llama_stack/apis/tools/__init__.py

@@ -0,0 +1,7 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+
+from .tools import *  # noqa: F401 F403