support for usdt

[mousehaohao]

USDT is a BTC token base on omni layer. As HDWallet is support BTC well, so support USDT is easy to implement.
To transfer USDT, we need to construct a transaction base on BTC, so we have two outputs.

1. transfer 546 Satoshi to destination
2. change to change address
3. a OP_RETURN output script to construct the USDT transaction.

Please check the code below:

public struct USDTTransactionBuilder: UtxoTransactionBuilderInterface {
    public init(usdtLockingScript: Data) {
        self.usdtLockingScript = usdtLockingScript
    }
    
    private let usdtLockingScript: Data
    
    public func build(destinations: [(address: Address, amount: UInt64)], utxos: [UnspentTransaction]) throws -> UnsignedTransaction {
        var outputs = try destinations.map { (address: Address, amount: UInt64) -> TransactionOutput in
            guard let lockingScript = Script(address: address)?.data else {
                throw TransactionBuildError.error("Invalid address type")
            }
            return TransactionOutput(value: amount, lockingScript: lockingScript)
        }
        let usdtTxOutput = TransactionOutput(value: 0, lockingScript: usdtLockingScript)
        outputs.append(usdtTxOutput) //add a new script output
        
        let unsignedInputs = utxos.map { TransactionInput(previousOutput: $0.outpoint, signatureScript: $0.output.lockingScript, sequence: UInt32.max) }
        let tx = Transaction(version: 1, inputs: unsignedInputs, outputs: outputs, lockTime: 0)
        return UnsignedTransaction(tx: tx, utxos: utxos)
    }
}

    static func generateUSDTLockingScript(amountInUSDT: Double) -> Data {
       //
        let converter = BitcoinConverter(bitcoinString: "\(amountInUSDT)")
        let amountInSatoshi = converter.inSatoshi
        
        var usdtData = Data()
        usdtData += [0x6a,0x14] //OP_RETURN
        usdtData += [0x6f,0x6d,0x6e,0x69] //OMNI
        usdtData += [0x00,0x00] //Transaction version
        usdtData += [0x00,0x00] // Transaction type
        usdtData += [0x00,0x00,0x00,0x1f] //Currency identifier
        ewLogger.debug("[USDT] amount data=\(amountInSatoshi.littleEndianData.toHexString())")
        usdtData += amountInSatoshi.littleEndianData
        
        return usdtData
    }

some references links:

• https://github.com/OmniLayer/spec#transfer-coins-simple-send
• https://blockchair.com/bitcoin/transaction/2e968f26c846426d7ac943236f91c1db484d8aa2f7a338be5ee194ca562fa3ec