feat: implement custom Stellar token/asset support for bets and payouts

[greatest0fallt1me]

Description:

Allow markets to accept and pay out in a configurable Stellar asset (e.g., USDC, custom token) in addition to or instead of native XLM, using Soroban token interface.

Requirements and Context:

• Must be secure, tested, and documented
• Should allow admin to set allowed token(s) per event or globally
• Should use token transfer and balance for locking and payouts
• Should validate token contract and decimals
• Should emit events with asset info
• Must handle approval/allowance if required by token
• Must not break XLM-native flow if still supported

Suggested Execution:

1. Fork the repo and create a branch

   git checkout -b feature/custom-token-support

2. Implement changes

   • Write contract: src/lib.rs (token parameter in place_bet, payouts)
   • Update tokens module: src/tokens.rs (multi-asset)
   • Update types and storage; add documentation and NatSpec-style comments

3. Test and commit

   • Run tests; cover XLM and custom token; insufficient balance
   • Include test output and security notes

Example Commit Message:

feat: implement custom Stellar token/asset support for bets and payouts

Guidelines:

• Minimum 95 percent test coverage
• Clear documentation
• Timeframe: 96 hours