[BOUNTY] Create Batch Deposit Interface

[sayanget]

Overview

This PR implements a gas-efficient interface for the PrivacyLayer Soroban contract, fulfilling the requirements of the [Create Batch Deposit Interface] bounty.

Technical Improvements

• Single Authentication: Users only need to authorize once for an entire batch of deposits, significantly reducing overhead.
• Batch Transfer: Funds for all commitments are transferred in a single token operation.
• Performance Optimized: Built on top of our previous Merkle zero-value optimizations, making large batches (e.g., 5-10 deposits) feasible within a single Stellar transaction.
• Indexer Friendly: Emits standard deposit events for each commitment within the batch to ensure existing indexers remain compatible.

API Changes

• Added to the main contract interface.

Testing

• Added comprehensive integration tests in to verify batch logic, total fund transfers, and event emissions.
• All 41 unit tests pass.

[sayanget]

Claim Information

• Bounty Task: [BOUNTY] Create Batch Deposit Interface
• Estimated ROI: 00+ USDC
• Payment Address (RTC): RTC89b248bde16082e0b7b9cb1b0ab29aa361a962c0