feat: Add custom User-Agent header to RestApiTool requests

Author: rcsantana777

src/google/adk/tools/openapi_tool/openapi_spec_parser/rest_api_tool.py

@@ -253,6 +253,10 @@ def _prepare_request_params(
     query_params: Dict[str, Any] = {}
     header_params: Dict[str, Any] = {}
     cookie_params: Dict[str, Any] = {}
+     
+     # CUSTOM USER-AGENT 
+    header_params["User-Agent"] = "google-adk/1.0" # You can adjust the version
+
 
     params_map: Dict[str, ApiParameter] = {p.py_name: p for p in parameters}