Use Base64VecU8 for tx_bytes in callbacks and storage

[olga24912]

Summary

• UTXO.tx_bytes and BTCPendingInfo.tx_bytes_with_sign switched from Vec<u8> to Base64VecU8 internally — compact base64 in JSON instead of arrays of numbers, saving gas on cross-contract callback args.
• Legacy view methods preserve the old format via UTXOView / BTCPendingInfoView (return tx_bytes as a JSON byte array). Off-chain clients keep working without changes.
• Added *_v2 view methods that return UTXO / BTCPendingInfo directly with base64 encoding.

State migration

Not required. Base64VecU8(pub Vec<u8>) is borsh-identical to Vec<u8>.