feat: add Vercel AI Gateway support (#274)

* 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

package.json

@@ -17,6 +17,7 @@
         "@ai-sdk/anthropic": "^2.0.44",
         "@ai-sdk/azure": "^2.0.69",
         "@ai-sdk/deepseek": "^1.0.30",
+        "@ai-sdk/gateway": "^2.0.21",
         "@ai-sdk/google": "^2.0.0",
         "@ai-sdk/openai": "^2.0.19",
         "@ai-sdk/react": "^2.0.107",