Feature/multi token support

[martinshub-tech]

this pr closes #948

Implement Escrow Multi-Token Support with Conversion Rates

Description

Currently, the Checkmate escrow contract only holds a single token per match. This PR implements multi-token support with automatic price conversion. Matches can now allow stakes in different tokens (e.g., USDC and native XLM) using a conversion rate verified by the Oracle.

Tasks Completed

• Match Struct Update: Added conversion_rate: i128 and token_b: Option<Address> fields to the Match struct.
• New Entrypoint: Implemented create_match_with_conversion(env, player1, player2, stake_amount, token_a, token_b, rate, game_id, platform).
• Oracle Integration:
  • Added set_rate, get_rate, and swap methods to the Oracle contract.
  • Added an on-chain verification step to ensure the creator's conversion rate is within ±5% of the oracle rate at match creation.
• Updated Lifecycle Flows:
  • Deposit: Calculates and transfers the correct token and amount depending on player role (Player 1 deposits token_a, Player 2 deposits token_b at the converted rate).
  • Payout: Winner receives the total prize pool converted back into their preferred token via Oracle swaps.
  • Refunds (Cancel/Expire): Deposits are returned to depositors in their originally staked tokens.
• Tests: Added a comprehensive test suite in multi_token.rs covering valid/invalid rate validation, multi-token deposits, payouts, swaps, and refunds.

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