Update ADK doc according to issue #1048 - 5

[adk-bot]

5. [Feature] Document OpenAPI Toolset enhancements.
   OpenAPIToolset and RestApiTool have been enhanced with new features: header_provider, ssl_verify, and tool_name_prefix.

   Proposed Change:
   In docs/tools-custom/openapi-tools.md, add a new section for advanced configuration.

   Current state:
   The documentation explains the basic usage of OpenAPIToolset.

   Proposed Change:
   Add the following section:

   ## Advanced Configuration
   
   ### Custom Headers with `header_provider`
   
   You can provide a callable to `header_provider` to dynamically add custom headers to every API request. The callable receives the `ReadonlyContext` and should return a dictionary of headers.
   
   ```python
   def my_header_provider(context: ReadonlyContext) -> dict[str, str]:
       return {"X-Request-ID": context.invocation_id}
   
   toolset = OpenAPIToolset(
       spec_dict=openapi_spec,
       header_provider=my_header_provider,
   )

   SSL Certificate Verification with ssl_verify

   You can configure SSL certificate verification for all tools in the toolset using the ssl_verify parameter. This is useful for environments with custom CA certificates.

   toolset = OpenAPIToolset(
       spec_dict=openapi_spec,
       ssl_verify="/path/to/ca.pem",
   )

   Tool Name Prefix with tool_name_prefix

   You can add a prefix to all tool names generated by the toolset using the tool_name_prefix parameter. This is useful for avoiding name collisions when using multiple OpenAPI specs.

   toolset = OpenAPIToolset(
       spec_dict=openapi_spec,
       tool_name_prefix="my_api",
   )

   
   **Reasoning**:
   These new features provide more flexibility and control over the OpenAPI tools, and they should be documented.
   
   **Reference**:
   - `src/google/adk/tools/openapi_tool/openapi_spec_parser/openapi_toolset.py`
   - `src/google/adk/tools/openapi_tool/openapi_spec_parser/rest_api_tool.py`