Add tokenExchange OAuth2 flow (OAuth 2.0 Token Exchange, RFC 8693) - #44

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v2, actions/setup-node@v2. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/