* feat: add Vercel AI Gateway support
- Updated environment configuration to include AI_GATEWAY_API_KEY for unified access to multiple AI providers.
- Added gateway provider to the list of supported AI providers in the codebase.
- Enhanced documentation to explain the usage of Vercel AI Gateway and its model format.
This change simplifies authentication and allows users to switch between providers seamlessly.
* Update package
@ai-sdk/gateway to latest version 2.0.21
