Add createPaymentIntent smart contract implementation

[MissBlue00]

Task

Implement the create_intent function in the Soroban payment_intent contract.

Details

• In contracts/src/payment_intent.rs
• Initialize contract storage with PaymentIntent data
• Validate caller is authorized
• Store amount, asset, sender, recipient, expiration
• Emit event on creation
• Return payment intent ID

Path

• contracts/src/payment_intent.rs

Test

Run cd contracts && cargo build to verify.