community[minor]: Improve Brave Search Tool, allow api key in env var (#30364)

- **Description:** 

- Make Brave Search Tool consistent with other tools and allow reading
its api key from `BRAVE_SEARCH_API_KEY` instead of having to pass the
api key manually (no breaking changes)
- Improve Brave Search Tool by storing api key in `SecretStr` instead of
plain `str`.
    - Add unit test for `BraveSearchWrapper`
    - Reflect the changes in the documentation
  - **Issue:** N/A
  - **Dependencies:** N/A
  - **Twitter handle:** ivan_brko

docs/docs/integrations/tools/brave_search.ipynb

@@ -43,12 +43,21 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 3,
+   "execution_count": null,
    "id": "5b14008a",
    "metadata": {},
    "outputs": [],
    "source": [
-    "tool = BraveSearch.from_api_key(api_key=api_key, search_kwargs={\"count\": 3})"
+    "tool = BraveSearch.from_api_key(api_key=api_key, search_kwargs={\"count\": 3})\n",
+    "\n",
+    "# or if you want to get the api key from environment variable BRAVE_SEARCH_API_KEY, and leave search_kwargs empty\n",
+    "# tool = BraveSearch()\n",
+    "\n",
+    "# or if you want to provide just the api key, and leave search_kwargs empty\n",
+    "# tool = BraveSearch.from_api_key(api_key=api_key)\n",
+    "\n",
+    "# or if you want to provide just the search_kwargs and read the api key from the BRAVE_SEARCH_API_KEY environment variable\n",
+    "# tool = BraveSearch.from_search_kwargs(search_kwargs={\"count\": 3})"
    ]
   },
   {