Add escrow deposit/release smart contract implementation

[MissBlue00]

Task

Implement the deposit and release functions in the Soroban escrow contract.

Details

• In contracts/src/escrow.rs
• deposit(): Hold funds in contract, record milestone conditions
• release(): Verify milestone met, release funds to recipient
• Handle refund to depositor if conditions not met
• Emit events for deposit, release, refund

Path

• contracts/src/escrow.rs

Test

Run cd contracts && cargo build to verify.