feat : Implement SignUp-TODO Input Validation & Refactor Auth Routes for Future Third-Party Authentication Integration

[Hamza1821]

Description:

• Completed the SignUp functionality with input validation for email format, nickname, and password strength.
• Moved all authentication-related routes to a separate auth.go file for better modularity and organization.
• Created an auth group in the server function to handle authentication routes like sign-up, sign-in, and email verification.
• Integrated environment variables for managing sensitive data like Cognito App Client ID and Secret.
• Prepares the foundation for integrating third-party authentication (like Google or GitHub) by implementing a flexible authentication system that can be extended in the future.

This PR addresses some key components of the issue [#9 Proposal: Implementing Third-Party Authentication for Debate AI Backend Using Go], laying the groundwork for future OAuth 2.0 integration.