[SMART CONTRACT] Implement Flash Loan Provider

[EDOHWARES]

Description
Develop a Soroban contract that provides flash loans to other contracts for arbitrage or liquidation.

Requirements and context

• Ensure repayment within the same transaction.
• Implement fee logic for the lender.
• Reentrancy protection.

Suggested execution

• Work in contracts/flash_loan/.
• Implement flash_loan(receiver, amount, data) function.

Example commit message
feat: implement flash loan provider contract