feat: Add Email-to-Wallet Contract

[0xZyrick]

This PR adds a Soroban smart contract that deterministically derives wallet addresses from user email strings.

Key Features

• derive_wallet_address_from_email(env, email) implementation
• Tests:
  • Same email → same address
  • Different emails → different addresses
  • Empty string → failure
• Utility added in utils.rs
• EmailToWalletContract exported in lib.rs

Fully tested (cargo test) and formatted (cargo fmt) ✅

Built for the NexaFX protocol’s identity layer.