Allow `allowed_tools` for MCP function calling

Hey there, I'm using Gemini's SDK with an MCP server and noticed there's no way to restrict which tools the model can call. Currently, I'm doing:

```
GenerateContentConfig(
system_instruction=system_message,
temperature=0,
tools=[mcp_client.session],
)
```

This gives access to all tools, but I'd like to allow only specific one, similar to allowed_tools in OpenAI's or Anthropic's SDKs. Would be great to have something like:

`tools=[mcp_client.session.with_allowed_tools(["tool_a", "tool_b"])]`

Thanks in advance!