Update ADK doc according to issue #1048 - 5

[adk-bot]

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",
)