OpenRouter OAuth

## Description

https://openrouter.ai/docs/guides/overview/auth/oauth

## Environment

- **OS:** [e.g. macOS 14.1, Ubuntu 22.04, Windows 11]
- **VT Code version:** [e.g. 0.31.0 or commit hash]
- **Rust version:** [e.g. rustc 1.93.0]

## Additional Context

Add any other context about the issue or question here.