feat: Implement Ethereum Wallet-Based User Registration

[seemenkina]

For the initial version of de-mls, user registration relied on a mock implementation for online registration. To align with Web3 principles and enhance user authentication, it is now essential to integrate support for Ethereum wallet-based user registration.

This feature will allow users to authenticate and register into group chat directly using their Ethereum wallets (e.g., MetaMask, WalletConnect-compatible wallets) instead of current solution (providing Ethereum private key). This improves security, user experience, and aligns with decentralized identity paradigms.

Scope:

• Integrate a Rust-compatible library or implement logic for Ethereum signature verification on the backend.
• Develop the necessary frontend components (e.g., a "Connect Wallet" button, UI for signing messages) to interact with popular Ethereum wallets.
• Update de-mls's user management system to support user identities based on Ethereum public addresses.
• Define the API endpoints required for the wallet connection, challenge generation, signature verification, and registration/login flow.
• Ensure secure handling of messages and signatures to prevent replay attacks and other vulnerabilities.
• Add comprehensive unit and integration tests for the new authentication flow.

Deliverables:

• Updated de-mls codebase with Ethereum wallet-based registration and authentication functionality.
• Clear API documentation for the new wallet integration endpoints.
• End-to-end tests covering the wallet registration/login flow.
• User-facing documentation or examples on how to register/login with an Ethereum wallet.