Custom fetch implementation

[ugo-studio]

This PR introduces a custom fetch implementation to enhance flexibility and compatibility in handling HTTP requests.

• Added support for custom fetch function.
• Converts Web Standard Response objects to Undici.Response for compatibility.
• Leverages Undici.request when no custom fetch function is provided.

Why This Change?
This custom implementation provides the following benefits:

• Flexibility: Developers can use their preferred fetch implementation (e.g., node-fetch, isomorphic-fetch, etc.).
• Improved Developer Experience: Simplifies HTTP request and response handling while maintaining robust error handling.