feat: implement Gas-less Transaction Support (EIP-712 Style Signatures)

[CollinsC1O]

Summary

feat: Gas-less Transaction Support — EIP-712 Style Signatures (#589)

Closes #589

Users sign contribution/registration intents off-chain via SorobanAuthorizationEntry; a relayer submits and pays fees on their behalf.

What's added

crowdfund_vault

• deposit_with_sig(user, project_id, amount, signature) — gasless contribution
• get_deposit_nonce(address) — nonce query for relayers
• GaslessDepositEvent, DepositNonce storage key, InvalidSignature error
• 5 new tests

project_registry

• register_project_with_sig(owner, project_id, name, signature) — gasless registration
• get_registration_nonce(address) — nonce query for relayers
• GaslessProjectRegisteredEvent, RegistrationNonce storage key, InvalidSignature error
• 4 new tests

Security

• Per-address nonces prevent replay attacks
• Auth scope bound to (fn_name, address, params, nonce) via require_auth_for_args

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[CollinsC1O]

@Cedarich All test passes sucessfully, I made sure they all do